fix: don't burn a job attempt when the browser is unavailable

A job's attempt is incremented at lease time, then the dispatcher acquires
the headless browser. When the browser was down or mid-restart, acquire()
failed and the error was ack_failed — so during an outage the whole pending
backlog was chewed to `dead` within minutes while workers hot-looped.

Add SyncOutcome::BrowserUnavailable: the dispatcher returns it instead of
propagating the acquire error, and the daemon releases the lease (refunding
the attempt, like the cancel/session paths) and backs off, so an outage is
treated as an infrastructure blip rather than per-job failures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/crawler/content.rs

@@ -71,6 +71,13 @@ pub enum SyncOutcome {
     /// Session probe failed mid-sync (avatar selector missing on the
     /// chapter page). Caller should abort the whole crawler run.
     SessionExpired,
+    /// The headless browser could not be acquired (down or mid-restart).
+    /// Produced only by the queue dispatcher (which calls `acquire()`); it is
+    /// an *infrastructure* outage, not a job failure, so the daemon returns the
+    /// job to `pending` WITHOUT burning a retry attempt. `sync_chapter_content`
+    /// itself never returns this — callers that already hold a lease can treat
+    /// it as unreachable.
+    BrowserUnavailable,
 }
 
 /// Per-chapter max fetch attempts when TOR is configured. `N = 3` means

backend/src/crawler/daemon.rs

@@ -66,6 +66,12 @@ const LEASE_DURATION: Duration = Duration::from_secs(60);
 /// the lease window leaves two missed-beat's slack before expiry.
 const LEASE_HEARTBEAT: Duration = Duration::from_secs(20);
 
+/// How long a worker waits after a `BrowserUnavailable` outcome before looping
+/// back to lease again. The job was released (not failed) so it stays pending;
+/// this backoff keeps the worker from hot-looping lease→acquire→release while
+/// the browser is down or mid-restart.
+const BROWSER_UNAVAILABLE_BACKOFF: Duration = Duration::from_secs(5);
+
 #[async_trait]
 pub trait MetadataPass: Send + Sync {
     async fn run(&self) -> anyhow::Result<pipeline::MetadataStats>;
@@ -502,6 +508,24 @@ impl WorkerContext {
                 self.status.poke();
                 let _ = jobs::release(&self.pool, lease.id, lease.lease_generation).await;
             }
+            Ok(Ok(SyncOutcome::BrowserUnavailable)) => {
+                // Infrastructure outage, not a job failure: the browser was
+                // down or mid-restart when the dispatcher tried to acquire it.
+                // Return the job to `pending` WITHOUT burning an attempt (like
+                // the cancel/session paths) so an outage doesn't chew the whole
+                // backlog to `dead`, then back off to avoid hot-looping while
+                // the browser recovers.
+                tracing::warn!(
+                    worker = self.id,
+                    lease_id = %lease.id,
+                    "worker: browser unavailable — released lease without burning an attempt"
+                );
+                let _ = jobs::release(&self.pool, lease.id, lease.lease_generation).await;
+                tokio::select! {
+                    _ = tokio::time::sleep(BROWSER_UNAVAILABLE_BACKOFF) => {}
+                    _ = self.cancel.cancelled() => {}
+                }
+            }
             Ok(Err(e)) => {
                 tracing::warn!(
                     worker = self.id,

backend/src/crawler/resync.rs

@@ -280,6 +280,12 @@ impl ResyncService for RealResyncService {
             SyncOutcome::SessionExpired => {
                 anyhow::bail!("source session expired — operator must refresh PHPSESSID")
             }
+            // Unreachable here: resync already holds its own browser lease and
+            // `sync_chapter_content` never acquires one, so it can't report the
+            // browser unavailable. Handled defensively for exhaustiveness.
+            SyncOutcome::BrowserUnavailable => {
+                anyhow::bail!("crawler browser unavailable")
+            }
         }
     }
 }