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>

internal/db/dbq/library_changes.sql.go

@@ -7,8 +7,25 @@ package dbq
 
 import (
 	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const deleteOldLibraryChanges = `-- name: DeleteOldLibraryChanges :execrows
+DELETE FROM library_changes WHERE changed_at < $1
+`
+
+// Removes library_changes rows older than the given cutoff. Run from
+// the sync compactor goroutine on a daily tick. Returns the number of
+// rows deleted so the worker can log meaningful progress.
+func (q *Queries) DeleteOldLibraryChanges(ctx context.Context, changedAt pgtype.Timestamptz) (int64, error) {
+	result, err := q.db.Exec(ctx, deleteOldLibraryChanges, changedAt)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
 const getLibraryChangesSince = `-- name: GetLibraryChangesSince :many
 SELECT id, entity_type, entity_id, op, changed_at
 FROM library_changes

internal/db/queries/library_changes.sql

@@ -14,3 +14,9 @@ SELECT COALESCE(MAX(id), 0)::BIGINT FROM library_changes;
 
 -- name: GetMinLibraryChangeCursor :one
 SELECT COALESCE(MIN(id), 0)::BIGINT FROM library_changes;
+
+-- name: DeleteOldLibraryChanges :execrows
+-- Removes library_changes rows older than the given cutoff. Run from
+-- the sync compactor goroutine on a daily tick. Returns the number of
+-- rows deleted so the worker can log meaningful progress.
+DELETE FROM library_changes WHERE changed_at < $1;