perf(feed): rewrite v_feed, bound feed_delta, server-time SSE cursor (H6,H7,M9)

H6: migration 006 replaces v_feed's double LEFT JOIN + COUNT(DISTINCT) (which
materialized a likes×comments Cartesian per upload) with correlated scalar
subqueries that each use their own index. Output columns are unchanged, so
feed + hashtag-filtered paths both benefit.

H7: feed_delta now applies the feed rate limit (keyed per user), caps results
at 200 rows, and clamps how far back a client `since` may reach (7 days). When
clamped or capped it returns reload_required=true; the SSE client turns that
into a full feed reload instead of streaming the whole gallery through the view
on every tab refocus.

M9: the SSE reconnect cursor is now advanced only from server timestamps (an
upload's created_at, seeded from the feed and updated on new-upload events and
delta responses), never the client clock — so a skewed phone clock can't drop
events missed while backgrounded.

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

backend/migrations/006_feed_view_perf.down.sql

@@ -0,0 +1,23 @@
+-- Restore the original join-based v_feed definition.
+CREATE OR REPLACE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    COUNT(DISTINCT l.user_id)    AS like_count,
+    COUNT(DISTINCT c.id)         AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+LEFT JOIN "like" l  ON l.upload_id = u.id
+LEFT JOIN comment c ON c.upload_id = u.id AND c.deleted_at IS NULL
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE
+GROUP BY u.id, usr.display_name, usr.is_banned, usr.uploads_hidden;

backend/migrations/006_feed_view_perf.up.sql

@@ -0,0 +1,26 @@
+-- H6: replace v_feed's double LEFT JOIN + COUNT(DISTINCT) (which materializes a
+-- likes×comments Cartesian per upload before de-duping) with correlated scalar
+-- subqueries. Each count now uses its own index (idx_like_upload /
+-- idx_comment_upload) and there is no GROUP BY. The output columns are
+-- unchanged, so every consumer keeps working.
+CREATE OR REPLACE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    (SELECT COUNT(*) FROM "like" l
+       WHERE l.upload_id = u.id)                              AS like_count,
+    (SELECT COUNT(*) FROM comment c
+       WHERE c.upload_id = u.id AND c.deleted_at IS NULL)     AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE;

backend/src/handlers/feed.rs

@@ -190,31 +190,70 @@ pub struct DeltaQuery {
 pub struct DeltaResponse {
     pub uploads: Vec<FeedUpload>,
     pub deleted_ids: Vec<Uuid>,
+    /// Set when the delta was clamped (too-old cursor) or hit the row cap — the
+    /// client should do a full feed reload instead of trusting the partial set.
+    pub reload_required: bool,
 }
 
+/// Hard cap on how many uploads one delta returns. Beyond this the client is
+/// told to reload rather than streaming the whole gallery through the view.
+const DELTA_LIMIT: i64 = 200;
+/// How far back a client-supplied `since` may reach. A tab backgrounded for days
+/// must not pull the entire event on reconnect.
+const DELTA_MAX_LOOKBACK_DAYS: i64 = 7;
+
 pub async fn feed_delta(
     State(state): State<AppState>,
     auth: AuthUser,
     Query(q): Query<DeltaQuery>,
 ) -> Result<Json<DeltaResponse>, AppError> {
+    // H7: feed_delta runs the (expensive) feed query and fires on every tab
+    // refocus, so it needs the same rate limit as feed(), keyed per user.
+    let rate_limits_on = config::get_bool(&state.pool, "rate_limits_enabled", true).await;
+    let feed_rate_on = config::get_bool(&state.pool, "feed_rate_enabled", true).await;
+    if rate_limits_on && feed_rate_on {
+        let rate_limit = config::get_usize(&state.pool, "feed_rate_per_min", 60).await;
+        if !state.rate_limiter.check(
+            format!("feed_delta:{}", auth.user_id),
+            rate_limit,
+            Duration::from_secs(60),
+        ) {
+            return Err(AppError::TooManyRequests(
+                "Zu viele Anfragen. Bitte warte kurz und versuche es erneut.".into(),
+                None,
+            ));
+        }
+    }
+
+    // Clamp the lookback server-side; signal a full reload if we had to.
+    let min_since = Utc::now() - chrono::Duration::days(DELTA_MAX_LOOKBACK_DAYS);
+    let clamped = q.since < min_since;
+    let since = if clamped { min_since } else { q.since };
+
     let rows = sqlx::query_as::<_, FeedRow>(
         "SELECT id, user_id, uploader_name, preview_path, thumbnail_path,
                 mime_type, caption, like_count, comment_count, created_at
          FROM v_feed
          WHERE event_id = $1 AND created_at > $2
-         ORDER BY created_at DESC",
+         ORDER BY created_at DESC
+         LIMIT $3",
     )
     .bind(auth.event_id)
-    .bind(q.since)
+    .bind(since)
+    .bind(DELTA_LIMIT + 1)
     .fetch_all(&state.pool)
     .await?;
 
+    let capped = rows.len() as i64 > DELTA_LIMIT;
+    let rows: Vec<FeedRow> = rows.into_iter().take(DELTA_LIMIT as usize).collect();
+    let reload_required = clamped || capped;
+
     let deleted_ids: Vec<(Uuid,)> = sqlx::query_as(
         "SELECT id FROM upload
          WHERE event_id = $1 AND deleted_at IS NOT NULL AND deleted_at > $2",
     )
     .bind(auth.event_id)
-    .bind(q.since)
+    .bind(since)
     .fetch_all(&state.pool)
     .await?;
 
@@ -234,6 +273,7 @@ pub async fn feed_delta(
     Ok(Json(DeltaResponse {
         uploads,
         deleted_ids: deleted_ids.into_iter().map(|r| r.0).collect(),
+        reload_required,
     }))
 }