feat(server): library_changes log table + sqlc queries

Append-only change log for library entities. Every mutation on
artists/albums/tracks/likes/playlists/playlist_tracks will write a row
in the same transaction as the mutation itself (wired in subsequent
commits). Powers the Flutter delta-sync endpoint (#357).

- 0025_library_changes migration (up + down)
- internal/db/queries/library_changes.sql (Insert, GetSince, MaxCursor, MinCursor)
- regenerated dbq from sqlc

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

internal/db/dbq/models.go

@@ -283,6 +283,14 @@ type GeneralLikesArtist struct {
 	LikedAt  pgtype.Timestamptz
 }
 
+type LibraryChange struct {
+	ID         int64
+	EntityType string
+	EntityID   string
+	Op         string
+	ChangedAt  pgtype.Timestamptz
+}
+
 type LidarrConfig struct {
 	ID                       int16
 	Enabled                  bool