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
+);