feat(crawler): live status via SSE instead of polling

Replace the dashboard's 5s polling with a Server-Sent Events stream:

- StatusHandle gains a tokio `watch` version bumped on every mutation;
  GET /admin/crawler/stream subscribes and pushes a composed snapshot
  immediately on connect, then on every status change (instant, no
  lost-wakeup) plus a 5s backstop for DB queue counts / browser phase.
- Non-status signals poke the notifier so they push immediately too:
  session-expired (worker), session update / clear-expired / browser
  restart (endpoints).
- compose_status is shared by the one-shot GET and the stream; the stream
  tolerates transient DB errors with a keep-alive comment instead of
  tearing down.

Frontend: the crawler page opens an EventSource on mount and closes it on
destroy, so the subscription is scoped to the active page (no global
subscription). A one-shot fetch still paints initial state / serves as a
fallback if SSE is blocked; a live/reconnecting indicator reflects the
connection. The existing reverse proxy already streams SSE (its abort
timer is cleared once response headers arrive), so no proxy change needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/api/admin/crawler.rs

@@ -6,9 +6,14 @@
 //! ([`crate::crawler::status`]) with DB-derived queue counts and the
 //! session/browser flags.
 
+use std::convert::Infallible;
+use std::time::Duration;
+
 use axum::extract::{Query, State};
+use axum::response::sse::{Event, KeepAlive, Sse};
 use axum::routing::{get, post};
 use axum::{Json, Router};
+use futures_util::stream::Stream;
 use serde::{Deserialize, Serialize};
 use serde_json::json;
 use uuid::Uuid;
@@ -21,9 +26,16 @@ use crate::error::{AppError, AppResult};
 use crate::repo;
 use crate::repo::crawler::{DeadJob, RequeueScope};
 
+/// Backstop recompose interval for the SSE stream. Phase/worker/session
+/// changes push instantly via the status `watch`; this only bounds the
+/// staleness of DB-derived queue counts and the browser phase when those
+/// change without an accompanying status poke.
+const SSE_BACKSTOP: Duration = Duration::from_secs(5);
+
 pub fn routes() -> Router<AppState> {
     Router::new()
         .route("/admin/crawler", get(get_status))
+        .route("/admin/crawler/stream", get(stream_status))
         .route("/admin/crawler/run", post(run_now))
         .route("/admin/crawler/browser/restart", post(restart_browser))
         .route("/admin/crawler/session", post(update_session))
@@ -75,10 +87,10 @@ fn browser_phase_str(p: RestartPhase) -> &'static str {
     }
 }
 
-async fn get_status(
-    State(state): State<AppState>,
-    _admin: RequireAdmin,
-) -> AppResult<Json<CrawlerStatusResponse>> {
+/// Compose a full status snapshot from the in-memory status, the
+/// browser/session flags, and a fresh DB queue-count query. Shared by the
+/// one-shot `get_status` and the SSE `stream_status`.
+async fn compose_status(state: &AppState) -> AppResult<CrawlerStatusResponse> {
     let (pending, running, dead) = repo::crawler::job_state_counts(&state.db).await?;
     let queue = QueueCounts {
         pending,
@@ -86,7 +98,7 @@ async fn get_status(
         dead,
     };
 
-    let resp = match state.crawler.as_ref() {
+    Ok(match state.crawler.as_ref() {
         None => CrawlerStatusResponse {
             daemon: "disabled",
             phase: None,
@@ -114,8 +126,64 @@ async fn get_status(
                 queue,
             }
         }
+    })
+}
+
+async fn get_status(
+    State(state): State<AppState>,
+    _admin: RequireAdmin,
+) -> AppResult<Json<CrawlerStatusResponse>> {
+    Ok(Json(compose_status(&state).await?))
+}
+
+// ---------------------------------------------------------------------------
+// GET /admin/crawler/stream — Server-Sent Events live status
+// ---------------------------------------------------------------------------
+
+/// Push live status to the dashboard instead of polling. Emits a snapshot
+/// immediately on connect, then on every status change (instant, via the
+/// `watch` notifier) and on a [`SSE_BACKSTOP`] tick (to refresh DB queue
+/// counts / browser phase that change without a status poke). The browser
+/// opens this only while the crawler page is mounted and closes it on
+/// navigate-away, so the subscription is scoped to the active page.
+async fn stream_status(
+    State(state): State<AppState>,
+    _admin: RequireAdmin,
+) -> Sse<impl Stream<Item = Result<Event, Infallible>>> {
+    // Subscribe before the first emit so no change between the initial
+    // snapshot and the first await is lost.
+    let rx = state.crawler.as_ref().map(|c| c.status.subscribe());
+
+    let stream = futures_util::stream::unfold(
+        (state, rx, true),
+        |(state, mut rx, first)| async move {
+            // After the first immediate emit, wait for a change or the
+            // backstop tick before recomposing.
+            if !first {
+                match rx.as_mut() {
+                    Some(rx) => {
+                        tokio::select! {
+                            _ = rx.changed() => {}
+                            _ = tokio::time::sleep(SSE_BACKSTOP) => {}
+                        }
+                    }
+                    None => tokio::time::sleep(SSE_BACKSTOP).await,
+                }
+            }
+            // Compose; on a transient DB error, emit a keep-alive comment
+            // rather than tearing down the stream.
+            let event = match compose_status(&state).await {
+                Ok(resp) => Event::default()
+                    .event("status")
+                    .json_data(&resp)
+                    .unwrap_or_else(|_| Event::default().comment("serialize error")),
+                Err(_) => Event::default().comment("status unavailable"),
             };
-    Ok(Json(resp))
+            Some((Ok(event), (state, rx, false)))
+        },
+    );
+
+    Sse::new(stream).keep_alive(KeepAlive::default())
 }
 
 // ---------------------------------------------------------------------------
@@ -165,6 +233,8 @@ async fn restart_browser(
 ) -> AppResult<Json<RestartResponse>> {
     let c = require_crawler(&state)?;
     let result = c.browser_manager.coordinated_restart(c.drain_deadline).await;
+    // Push the post-restart browser phase to live subscribers immediately.
+    c.status.poke();
     repo::admin_audit::insert(
         &state.db,
         admin.0.id,
@@ -215,6 +285,8 @@ async fn update_session(
     // Relaunch the browser so on_launch re-injects the new cookie and
     // re-probes — the restart's success IS the session-validity signal.
     let probe = c.browser_manager.coordinated_restart(c.drain_deadline).await;
+    // Session + browser state changed — push to live subscribers.
+    c.status.poke();
     repo::admin_audit::insert(
         &state.db,
         admin.0.id,
@@ -247,6 +319,8 @@ async fn clear_session_expired(
 ) -> AppResult<Json<ClearExpiredResponse>> {
     let c = require_crawler(&state)?;
     c.session.clear_expired();
+    // session.expired flipped — push to live subscribers.
+    c.status.poke();
     repo::admin_audit::insert(
         &state.db,
         admin.0.id,

backend/src/crawler/daemon.rs

@@ -447,6 +447,8 @@ impl WorkerContext {
                     "session expired — workers will idle until restart"
                 );
                 self.session_expired.store(true, Ordering::Release);
+                // Push the session-expired flip to live status subscribers.
+                self.status.poke();
                 let _ = jobs::release(&self.pool, lease.id).await;
             }
             Ok(Err(e)) => {

backend/src/crawler/status.rs

@@ -16,7 +16,7 @@ use std::sync::Arc;
 
 use chrono::{DateTime, Utc};
 use serde::Serialize;
-use tokio::sync::RwLock;
+use tokio::sync::{watch, RwLock};
 use uuid::Uuid;
 
 use crate::crawler::pipeline::MetadataStats;
@@ -84,29 +84,62 @@ impl CrawlerStatus {
 
 /// Cloneable handle the daemon tasks use to publish status. Cheap to clone
 /// (`Arc`). All writers funnel through the helper methods so locking stays
-/// localised.
+/// localised. Every mutation bumps a `watch` version so SSE subscribers
+/// get pushed an update instead of polling.
 #[derive(Clone)]
-pub struct StatusHandle(Arc<RwLock<CrawlerStatus>>);
+pub struct StatusHandle {
+    inner: Arc<RwLock<CrawlerStatus>>,
+    /// Monotonic version bumped on every change. SSE handlers
+    /// `subscribe()` and `await .changed()` for instant pushes; `watch`
+    /// has no lost-wakeup so a change between snapshots is never missed.
+    version: Arc<watch::Sender<u64>>,
+}
 
 impl StatusHandle {
     pub fn new(num_workers: usize) -> Self {
-        Self(Arc::new(RwLock::new(CrawlerStatus::new(num_workers))))
+        let (version, _rx) = watch::channel(0u64);
+        Self {
+            inner: Arc::new(RwLock::new(CrawlerStatus::new(num_workers))),
+            version: Arc::new(version),
+        }
+    }
+
+    fn bump(&self) {
+        self.version.send_modify(|v| *v = v.wrapping_add(1));
+    }
+
+    /// A receiver whose `.changed()` resolves on the next status change.
+    pub fn subscribe(&self) -> watch::Receiver<u64> {
+        self.version.subscribe()
+    }
+
+    /// Signal a change without mutating in-memory state — used when an
+    /// *external* signal the live snapshot reflects (browser phase,
+    /// session-expired flag, queue counts) has changed, so subscribers
+    /// recompose promptly.
+    pub fn poke(&self) {
+        self.bump();
     }
 
     pub async fn set_phase(&self, phase: Phase) {
-        self.0.write().await.phase = phase;
+        self.inner.write().await.phase = phase;
+        self.bump();
     }
 
     pub async fn set_worker(&self, id: usize, state: WorkerState) {
-        let mut s = self.0.write().await;
+        {
+            let mut s = self.inner.write().await;
             if let Some(slot) = s.workers.get_mut(id) {
                 *slot = state;
             }
         }
+        self.bump();
+    }
 
     /// Record a finished metadata pass. Stamps `at` with `now`.
     pub async fn record_pass(&self, stats: &MetadataStats, at: DateTime<Utc>) {
-        let mut s = self.0.write().await;
+        {
+            let mut s = self.inner.write().await;
             s.last_pass = LastPass {
                 at: Some(at),
                 discovered: stats.discovered,
@@ -115,15 +148,18 @@ impl StatusHandle {
                 mangas_failed: stats.mangas_failed,
             };
         }
+        self.bump();
+    }
 
     /// Seed the last-pass summary from a persisted `crawler_state` value on
     /// startup so the dashboard isn't blank until the first tick.
     pub async fn set_last_pass(&self, last: LastPass) {
-        self.0.write().await.last_pass = last;
+        self.inner.write().await.last_pass = last;
+        self.bump();
     }
 
     pub async fn snapshot(&self) -> CrawlerStatus {
-        self.0.read().await.clone()
+        self.inner.read().await.clone()
     }
 }
 
@@ -159,4 +195,16 @@ mod tests {
         assert_eq!(snap.last_pass.upserted, 3);
         assert_eq!(snap.last_pass.at, Some(at));
     }
+
+    #[tokio::test]
+    async fn subscribe_resolves_on_mutation_and_poke() {
+        let h = StatusHandle::new(1);
+        let mut rx = h.subscribe();
+        // A mutation wakes the subscriber.
+        h.set_phase(Phase::WalkingList).await;
+        rx.changed().await.unwrap();
+        // A bare poke (external signal) also wakes it.
+        h.poke();
+        rx.changed().await.unwrap();
+    }
 }

backend/tests/api_admin_crawler.rs

@@ -7,8 +7,11 @@
 
 mod common;
 
+use std::time::Duration;
+
 use axum::http::StatusCode;
 use axum::Router;
+use http_body_util::BodyExt;
 use serde_json::json;
 use sqlx::PgPool;
 use tower::ServiceExt;
@@ -121,6 +124,70 @@ async fn control_endpoints_return_503_when_daemon_disabled(pool: PgPool) {
     }
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn status_stream_requires_admin(pool: PgPool) {
+    let h = harness(pool);
+    // Unauthenticated → 401.
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get("/api/v1/admin/crawler/stream"))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::UNAUTHORIZED);
+    // Non-admin → 403.
+    let (_u, cookie) = register_user(&h.app).await;
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get_with_cookie("/api/v1/admin/crawler/stream", &cookie))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::FORBIDDEN);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn status_stream_emits_initial_event(pool: PgPool) {
+    let h = harness(pool.clone());
+    let cookie = seed_admin(&pool, &h.app).await;
+
+    let resp = h
+        .app
+        .clone()
+        .oneshot(get_with_cookie("/api/v1/admin/crawler/stream", &cookie))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let ct = resp
+        .headers()
+        .get(axum::http::header::CONTENT_TYPE)
+        .and_then(|v| v.to_str().ok())
+        .unwrap_or_default()
+        .to_string();
+    assert!(ct.starts_with("text/event-stream"), "content-type was {ct:?}");
+
+    // Accumulate frames (the immediate snapshot may arrive split across
+    // frames) until the status payload appears, with an overall timeout so
+    // the never-ending stream can't hang the test.
+    let mut body = resp.into_body();
+    let mut acc = String::new();
+    let deadline = tokio::time::timeout(Duration::from_secs(5), async {
+        loop {
+            let Some(frame) = body.frame().await else { break };
+            if let Ok(data) = frame.expect("frame ok").into_data() {
+                acc.push_str(&String::from_utf8_lossy(&data));
+                if acc.contains("\"daemon\"") {
+                    break;
+                }
+            }
+        }
+    })
+    .await;
+    assert!(deadline.is_ok(), "did not receive status within 5s; got: {acc:?}");
+    assert!(acc.contains("\"daemon\""), "missing status payload: {acc}");
+    assert!(acc.contains("status"), "missing SSE event name: {acc}");
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn mutating_endpoints_reject_non_admin(pool: PgPool) {
     let h = harness(pool);

frontend/src/lib/api/admin.test.ts

@@ -18,6 +18,7 @@ import {
     resyncManga,
     resyncChapter,
     getCrawlerStatus,
+    crawlerStatusStreamUrl,
     runCrawlerPass,
     restartCrawlerBrowser,
     updateCrawlerSession,
@@ -356,6 +357,10 @@ describe('admin crawler api client', () => {
         queue: { pending: 2, running: 1, dead: 4 }
     };
 
+    it('crawlerStatusStreamUrl points at the SSE endpoint under the API base', () => {
+        expect(crawlerStatusStreamUrl()).toMatch(/\/v1\/admin\/crawler\/stream$/);
+    });
+
     it('getCrawlerStatus GETs /v1/admin/crawler', async () => {
         fetchSpy.mockResolvedValueOnce(ok(statusFixture));
         const s = await getCrawlerStatus();

frontend/src/lib/api/admin.ts

@@ -3,7 +3,7 @@
 // won't reach these routes). 403s thrown here propagate up to the
 // /admin layout, which renders the framework error page.
 
-import { request, type Page } from './client';
+import { request, apiUrl, type Page } from './client';
 import type { User } from './auth';
 import type { MangaDetail } from './mangas';
 import type { Chapter } from './chapters';
@@ -248,6 +248,14 @@ export async function getCrawlerStatus(): Promise<CrawlerStatus> {
     return request<CrawlerStatus>('/v1/admin/crawler');
 }
 
+/** URL of the Server-Sent Events live-status stream. Open with
+ *  `new EventSource(...)` while the crawler page is mounted and close it on
+ *  navigate-away so the subscription is scoped to the active page. Each
+ *  message is a named `status` event whose `data` is a {@link CrawlerStatus}. */
+export function crawlerStatusStreamUrl(): string {
+    return apiUrl('/v1/admin/crawler/stream');
+}
+
 /** POST /v1/admin/crawler/run — trigger an out-of-cycle metadata pass. */
 export async function runCrawlerPass(): Promise<{ started: boolean }> {
     return request('/v1/admin/crawler/run', { method: 'POST' });

frontend/src/lib/api/client.ts

@@ -12,6 +12,15 @@ export function fileUrl(key: string): string {
     return `${BASE}/v1/files/${key}`;
 }
 
+/**
+ * Builds an API URL for non-`fetch` consumers (e.g. `EventSource` for SSE),
+ * applying the same `VITE_API_BASE` prefix as `request()`. `path` is the
+ * route after the base, e.g. `/v1/admin/crawler/stream`.
+ */
+export function apiUrl(path: string): string {
+    return `${BASE}${path}`;
+}
+
 export class ApiError extends Error {
     constructor(
         public readonly status: number,

frontend/src/routes/admin/crawler/+page.svelte

@@ -4,6 +4,7 @@
     import Pager from '$lib/components/Pager.svelte';
     import {
         getCrawlerStatus,
+        crawlerStatusStreamUrl,
         runCrawlerPass,
         restartCrawlerBrowser,
         updateCrawlerSession,
@@ -19,7 +20,8 @@
     let status: CrawlerStatus | null = $state(null);
     let error: string | null = $state(null);
     let notice: string | null = $state(null);
-    let timer: ReturnType<typeof setInterval> | null = null;
+    let live = $state(false);
+    let source: EventSource | null = null;
     let busy = $state(false);
 
     // Dead jobs
@@ -58,13 +60,40 @@
         }
     }
 
+    // Live updates via Server-Sent Events instead of polling. The
+    // EventSource is opened on mount and closed on destroy, so the
+    // subscription exists only while this page is showing live data.
+    function openStream() {
+        const es = new EventSource(crawlerStatusStreamUrl(), { withCredentials: true });
+        es.addEventListener('status', (e) => {
+            try {
+                status = JSON.parse((e as MessageEvent).data) as CrawlerStatus;
+                error = null;
+                live = true;
+            } catch {
+                // ignore a malformed frame; the next one will replace it
+            }
+        });
+        es.onopen = () => {
+            live = true;
+        };
+        es.onerror = () => {
+            // The browser auto-reconnects; reflect the gap in the UI.
+            live = false;
+        };
+        source = es;
+    }
+
     onMount(() => {
+        // One-shot fetch for instant initial paint + resilience if SSE is
+        // blocked; the stream then drives subsequent updates.
         refresh();
         loadDeadJobs();
-        timer = setInterval(refresh, 5000);
+        openStream();
     });
     onDestroy(() => {
-        if (timer) clearInterval(timer);
+        source?.close();
+        source = null;
     });
 
     async function withBusy(label: string, fn: () => Promise<void>) {
@@ -188,7 +217,12 @@
     const deadTotalPages = $derived(Math.max(1, Math.ceil(deadTotal / DEAD_LIMIT)));
 </script>
 
-<h1>Crawler</h1>
+<div class="titlebar">
+    <h1>Crawler</h1>
+    <span class="livedot" class:on={live} title={live ? 'Live (SSE)' : 'Reconnecting…'}>
+        {live ? '● live' : '○ reconnecting…'}
+    </span>
+</div>
 
 {#if error}
     <p class="error" role="alert">{error}</p>
@@ -403,7 +437,20 @@
 
 <style>
     h1 {
-        margin: 0 0 var(--space-4) 0;
+        margin: 0;
+    }
+    .titlebar {
+        display: flex;
+        align-items: baseline;
+        gap: var(--space-3);
+        margin-bottom: var(--space-4);
+    }
+    .livedot {
+        font-size: var(--font-sm);
+        color: var(--text-muted);
+    }
+    .livedot.on {
+        color: var(--success, #0a7d2c);
     }
     h2 {
         margin: 0 0 var(--space-3) 0;