fix: reap expired sessions with a periodic background sweep

The sessions table only grew — find_active ignored expired rows but nothing
deleted them, so lapsed sessions accumulated indefinitely.

- repo::session::delete_expired: indexed DELETE (sessions_expires_idx) returning
  the reaped count.
- app::build spawns a detached hourly reaper (SESSION_GC_INTERVAL) that calls it,
  independent of crawler/analysis config.

Test: delete_expired_removes_only_lapsed_sessions.

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

backend/src/app.rs

@@ -277,6 +277,11 @@ impl DaemonReloader for Supervisors {
     }
 }
 
+/// How often the background reaper sweeps expired sessions. Hourly is ample:
+/// the sweep is a single indexed DELETE and expired rows are already invisible
+/// to auth, so this is purely storage hygiene.
+const SESSION_GC_INTERVAL: std::time::Duration = std::time::Duration::from_secs(3600);
+
 pub async fn build(config: Config) -> anyhow::Result<AppHandle> {
     let db = PgPoolOptions::new()
         .max_connections(config.db.max_connections)
@@ -337,6 +342,27 @@ pub async fn build(config: Config) -> anyhow::Result<AppHandle> {
         tracing::info!("analysis worker disabled");
     }
 
+    // Periodic reaper for lapsed sessions. `find_active` already ignores
+    // expired rows, so this only reclaims storage — without it the table grows
+    // unbounded as sessions lapse. Detached and best-effort: a failed sweep is
+    // logged and retried next tick. Runs regardless of crawler/analysis config
+    // since sessions exist in every deployment.
+    {
+        let db = db.clone();
+        tokio::spawn(async move {
+            let mut ticker = tokio::time::interval(SESSION_GC_INTERVAL);
+            ticker.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);
+            loop {
+                ticker.tick().await;
+                match repo::session::delete_expired(&db).await {
+                    Ok(0) => {}
+                    Ok(n) => tracing::info!(reaped = n, "session gc: removed expired sessions"),
+                    Err(e) => tracing::warn!(?e, "session gc sweep failed; retrying next tick"),
+                }
+            }
+        });
+    }
+
     let auth_limiter = Arc::new(AuthRateLimiter::new(config.auth.rate_limit));
     let state = AppState {
         db,

backend/src/repo/session.rs

@@ -65,3 +65,15 @@ pub async fn delete_by_token_hash(pool: &PgPool, token_hash: &[u8]) -> AppResult
         .await?;
     Ok(())
 }
+
+/// Delete every session whose `expires_at` has passed, returning the number
+/// reaped. `find_active` already refuses expired sessions, so this only
+/// reclaims storage — running it on any schedule (or not at all) is safe.
+/// Backed by `sessions_expires_idx` (0002). Called by the periodic reaper in
+/// `app::build`.
+pub async fn delete_expired(pool: &PgPool) -> AppResult<u64> {
+    let result = sqlx::query("DELETE FROM sessions WHERE expires_at <= now()")
+        .execute(pool)
+        .await?;
+    Ok(result.rows_affected())
+}