step 2: Add UUID columns with backfill and dual-write support

- Add migration 000007: Add UUID columns to soundtrack and song with backfill
- Update InsertSoundtrack and InsertSoundtrackWithExistingId to accept UUID
- Update AddSong to accept UUID
- Add dual-write: Go code now generates UUIDs for new records
- Add uuid and pgtype imports

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/db/repository/song.sql.go

@@ -27,19 +27,21 @@ func (q *Queries) AddHashToSong(ctx context.Context, arg AddHashToSongParams) er
 }
 
 const addSong = `-- name: AddSong :exec
-INSERT INTO song(soundtrack_id, song_name, path, file_name, hash) VALUES ($1, $2, $3, $4, $5)
+INSERT INTO song(uuid, soundtrack_id, song_name, path, file_name, hash) VALUES ($1, $2, $3, $4, $5, $6)
 `
 
 type AddSongParams struct {
-	SoundtrackID int32   `json:"soundtrack_id"`
-	SongName     string  `json:"song_name"`
-	Path         string  `json:"path"`
-	FileName     *string `json:"file_name"`
-	Hash         string  `json:"hash"`
+	Uuid         pgtype.UUID `json:"uuid"`
+	SoundtrackID int32       `json:"soundtrack_id"`
+	SongName     string      `json:"song_name"`
+	Path         string      `json:"path"`
+	FileName     *string     `json:"file_name"`
+	Hash         string      `json:"hash"`
 }
 
 func (q *Queries) AddSong(ctx context.Context, arg AddSongParams) error {
 	_, err := q.db.Exec(ctx, addSong,
+		arg.Uuid,
 		arg.SoundtrackID,
 		arg.SongName,
 		arg.Path,