fix: address code review findings

- Store cached_at in canonical UTC layout so the cache TTL cutoff comparison is a valid time ordering (previously go-sqlite3 serialized time.Time as RFC3339, making the space-separated cutoff match only by ASCII accident; same-day expired entries were falsely served as fresh).

- Remove stale no-op scanner config keys (ignore_bootlegs, include_compilations) from config.yaml.example and docs; these fields were removed from ScannerConfig but left in configs, silently doing nothing.

internal/musicbrainz/sync.go

@@ -137,7 +137,7 @@ func SyncArtistDiscography(
 
 		if _, err := tx.Exec(
 			"INSERT INTO external_releases (rgid, artist_id, title, type, release_date, is_ignored, cached_at) VALUES (?, ?, ?, ?, ?, ?, ?)",
-			ext.RGID, ext.ArtistID, ext.Title, ext.Type, ext.ReleaseDate, ext.IsIgnored, ext.CachedAt,
+			ext.RGID, ext.ArtistID, ext.Title, ext.Type, ext.ReleaseDate, ext.IsIgnored, database.FormatCachedAt(ext.CachedAt),
 		); err != nil {
 			return nil, fmt.Errorf("sync artist discography: insert release %s: %w", rg.ID, err)
 		}