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>
