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/schemas/_libraries.sql

@@ -0,0 +1,6 @@
+CREATE TABLE IF NOT EXISTS libraries (
+    id         INTEGER PRIMARY KEY,
+    name       TEXT NOT NULL,
+    path       TEXT NOT NULL UNIQUE,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);

backend/database/sql/schemas/audio_files.sql

@@ -10,9 +10,14 @@ CREATE TABLE IF NOT EXISTS audio_files (
   bitrate int NOT NULL DEFAULT 0,
   file_size int NOT NULL DEFAULT 0,
   basename text NOT NULL DEFAULT '',
+  library_id int NOT NULL DEFAULT 0,
   FOREIGN KEY(file_type_id) REFERENCES file_types(id),
-  FOREIGN KEY(recording_id) REFERENCES recordings(id)
+  FOREIGN KEY(recording_id) REFERENCES recordings(id),
+  FOREIGN KEY(library_id) REFERENCES libraries(id)
 );
 
 CREATE INDEX IF NOT EXISTS idx_audio_files_recording_id
     ON audio_files(recording_id);
+
+CREATE INDEX IF NOT EXISTS idx_audio_files_library_id
+    ON audio_files(library_id);

backend/database/sql/schemas/playlist_tracks.sql

@@ -1,10 +1,16 @@
 CREATE TABLE IF NOT EXISTS playlist_tracks (
     id INTEGER PRIMARY KEY,
     playlist_id INTEGER NOT NULL,
-    audio_file_id INTEGER NOT NULL,
+    audio_file_id INTEGER,
     position INTEGER NOT NULL,
+    phantom_title TEXT,
+    phantom_artist TEXT,
+    phantom_album TEXT,
+    phantom_duration_ms INTEGER,
+    phantom_genre TEXT,
+    phantom_cover_art_path TEXT,
     FOREIGN KEY(playlist_id) REFERENCES playlists(id) ON DELETE CASCADE,
-    FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE CASCADE
+    FOREIGN KEY(audio_file_id) REFERENCES audio_files(id) ON DELETE SET NULL
 );
 
 CREATE INDEX IF NOT EXISTS idx_playlist_tracks_playlist_id

backend/database/sql/schemas/track_metadata_view.sql

@@ -22,7 +22,8 @@ SELECT
     af.bit_depth,
     af.channels,
     af.bitrate,
-    af.file_size
+    af.file_size,
+    af.library_id
 FROM audio_files af
 LEFT JOIN recordings r ON af.recording_id = r.id
 LEFT JOIN artist_credit ac ON r.artist_credit_id = ac.id