feat(10-01): update SQL schema files for multi-library fresh installs

- Create _libraries.sql with libraries table (name, path, created_at)
- Add library_id FK column and index to audio_files.sql
- Update playlist_tracks.sql with nullable audio_file_id, SET NULL FK, and 6 phantom columns
- Add af.library_id to track_metadata VIEW
- Regenerate sqlc code for updated schemas
- Fix playlist.go to use sql.NullInt64 for nullable audio_file_id

backend/database/sql/sqlcgen/models.go

@@ -37,6 +37,7 @@ type AudioFile struct {
 	Bitrate            int64
 	FileSize           int64
 	Basename           string
+	LibraryID          int64
 }
 
 type CoverArt struct {
@@ -56,6 +57,13 @@ type Genre struct {
 	Name string
 }
 
+type Library struct {
+	ID        int64
+	Name      string
+	Path      string
+	CreatedAt time.Time
+}
+
 type PlayerState struct {
 	ID                  int64
 	Volume              int64
@@ -72,10 +80,16 @@ type Playlist struct {
 }
 
 type PlaylistTrack struct {
-	ID          int64
-	PlaylistID  int64
-	AudioFileID int64
-	Position    int64
+	ID                  int64
+	PlaylistID          int64
+	AudioFileID         sql.NullInt64
+	Position            int64
+	PhantomTitle        sql.NullString
+	PhantomArtist       sql.NullString
+	PhantomAlbum        sql.NullString
+	PhantomDurationMs   sql.NullInt64
+	PhantomGenre        sql.NullString
+	PhantomCoverArtPath sql.NullString
 }
 
 type Queue struct {
@@ -155,4 +169,5 @@ type TrackMetadatum struct {
 	Channels           int64
 	Bitrate            int64
 	FileSize           int64
+	LibraryID          int64
 }