feat(#357): library_changes retention compactor

Closes the last deferred follow-up from #357. The library_changes
table is the append-only change log that drives /api/library/sync's
delta semantics — every mutation (scanner upsert, like, playlist
edit, track delete) writes one row. Without a retention policy the
table grows unbounded; the original migration (0025) called out the
follow-up explicitly.

New goroutine: sync.Compactor runs daily, deletes rows where
changed_at < now - 30 days. Logs a row count when non-zero so
operators can see compaction activity in the journal. First tick
fires on startup so a process that hasn't been compacted in a
while catches up immediately.

30-day retention matches the offline-mode spec
(docs/superpowers/specs/2026-05-09-flutter-offline-mode-design.md).
Clients with a cursor older than that hit the existing 410 fallback
path and resync from scratch.

Imported as syncpkg in main.go to follow the existing convention
(see internal/library/scanner.go).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -25,6 +25,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/server"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/similarity"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/subsonic"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 func main() {
@@ -159,6 +160,13 @@ func run() error {
 	lidarrReconciler := lidarrrequests.NewReconciler(pool, lidarrCfg, logger.With("component", "lidarr"), bus)
 	go lidarrReconciler.Run(ctx)
 
+	// library_changes compactor (#357 follow-up). Daily tick; deletes
+	// rows older than the configured retention so the change-log table
+	// doesn't grow unbounded. Clients that drop offline longer than
+	// retention hit the /api/library/sync 410 fallback and resync.
+	libraryChangesCompactor := syncpkg.NewCompactor(pool, logger.With("component", "library_changes_compactor"))
+	go libraryChangesCompactor.Run(ctx)
+
 	// Per-user system-playlist scheduler (#392 Half B). Fires each
 	// active user's daily build at 03:00 in their stored timezone.
 	// Replaces the 24h-anchored cron loop (removed in the next commit