feat(server): drift audit batch 7 — periodic GC worker for 5 lifecycle gaps

New `internal/gc` package with a single Worker that runs all five
lifecycle / retention sweeps from the 2026-06-02 drift audit on a
1-hour tick. Each sweep is small, idempotent (re-running on
already-clean rows is a no-op), and logs its affected-row count.

Sweeps (Scribe parent #552):

- **#566** GcCloseStalePlayEvents — play_events rows opened > 24h
  ago that never got a play_ended (client crash, network drop).
  Synthesizes ended_at from duration_played_ms when known, falls
  back to now() so the row stops looking "open" to downstream
  filters (ended_at IS NULL).

- **#565** GcClosePlaySessionsWithNoRecentEvents — play_sessions
  with last_event_at older than 6h get ended_at = last_event_at
  ("user moved on"); empty sessions older than 1h get closed
  too (stale handshakes from clients that never recorded a play).
  The audit caught that the column was added but never populated
  by any writer — every session row was "open" forever, breaking
  downstream dedup queries that assume closed semantics.

- **#567** GcExpireScrobbleQueueFailedRows — drops scrobble_queue
  rows in status='failed' older than 14 days. The worker stops
  retrying after maxAttempts so these otherwise accumulate
  forever on a persistent ListenBrainz outage / revoked token.

- **#574** GcResetStuckSystemPlaylistRuns — flips
  system_playlist_runs.in_flight back to false on rows whose
  last_run_at is older than 10 minutes. Catches goroutine-panic
  wedges where the generator died between SET in_flight=true and
  SET in_flight=false; the duplicate-prevention check refuses to
  start a fresh regen while in_flight, so a stuck row would
  otherwise deadlock all future regens for that user. Records
  "stuck-row auto-reset by gc" in last_error so the operator can
  tell auto-reset from a recent real failure.

- **#575** GcDeleteExpiredPasswordResets — deletes expired
  password_resets rows. Unused expired rows go after a 1h grace
  (gives the operator time to debug an active reset attempt);
  used rows are kept 7 days for audit.

Wiring:
- main.go `go gcWorker.Run(ctx)` alongside the other periodic
  workers (scrobble, similarity, lidarr).
- tickOnce fires once at start so a freshly-deployed server does
  its initial sweep without waiting a full tick, matching the
  scrobble worker pattern.
- Errors per sweep are logged but do NOT abort the remaining
  ones — a transient pgx error from one query shouldn't prevent
  the others from running.

Tests:
- 4 integration tests, one per UPDATE/DELETE sweep, that seed
  rows-to-sweep + rows-to-leave-alone and assert the right rows
  changed state. Skip unless MINSTREL_TEST_DATABASE_URL is set
  (mirrors the api package pattern).
- Empty-tables no-op smoke test.
- Run() cancellation honoured (no spinning goroutine at
  test-runner exit).

That's all five remaining server-side lifecycle findings from the
audit. The Android LOCAL_USER_ID hardcode (#576) is a separate
refactor that needs auth-store wiring and stays in the queue.

cmd/minstrel/main.go

@@ -15,6 +15,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/eventbus"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/gc"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarr"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarrconfig"
@@ -161,6 +162,16 @@ func run() error {
 	similarityWorker := similarity.NewWorker(pool, listenbrainz.NewClient(), logger.With("component", "similarity"))
 	go similarityWorker.Run(ctx)
 
+	// Start the GC worker. Runs every 1h and sweeps lifecycle tables
+	// that have no writer-side close path or retention policy:
+	// orphan play_events, stale play_sessions, expired
+	// scrobble_queue failures, stuck system_playlist_runs, expired
+	// password_resets. Each sweep is idempotent — a row that's
+	// already clean is a no-op. Addresses drift audit findings
+	// #565 #566 #567 #574 #575 (Scribe parent #552).
+	gcWorker := gc.NewWorker(pool, logger.With("component", "gc"))
+	go gcWorker.Run(ctx)
+
 	// Start the Lidarr reconciler worker. Per spec §M5a, polls pending Lidarr
 	// import requests and reconciles them against the library. Short-circuits
 	// to no-op when lidarr_config.enabled = false.