feat(10-02): add sqlc queries for libraries and update playlist queries for phantom support

- Create libraries.sql with 7 CRUD queries (create, get, get-by-path, list, update, delete, count)
- Update playlists.sql: AddPlaylistTrack now accepts 9 params including phantom metadata
- Update playlist queries to use LEFT JOIN for nullable audio_file_id
- Add GetTrackPhantomMetadata helper query for eager phantom population
- Add is_phantom computed column to metadata queries
- Add GetAudioFilesByLibrary and CountAudioFilesByLibrary queries
- Regenerate all sqlc code

backend/database/sql/queries/libraries.sql

@@ -0,0 +1,21 @@
+-- name: CreateLibrary :one
+INSERT INTO libraries (name, path) VALUES (?, ?)
+RETURNING *;
+
+-- name: GetLibrary :one
+SELECT * FROM libraries WHERE id = ? LIMIT 1;
+
+-- name: GetLibraryByPath :one
+SELECT * FROM libraries WHERE path = ? LIMIT 1;
+
+-- name: GetAllLibraries :many
+SELECT * FROM libraries ORDER BY name;
+
+-- name: UpdateLibraryName :exec
+UPDATE libraries SET name = ? WHERE id = ?;
+
+-- name: DeleteLibrary :exec
+DELETE FROM libraries WHERE id = ?;
+
+-- name: CountLibraries :one
+SELECT COUNT(*) AS count FROM libraries;