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/queries/song.sql

@@ -5,7 +5,7 @@ DELETE FROM song;
 DELETE FROM song WHERE soundtrack_id = $1;
 
 -- 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);
 
 -- name: CheckSong :one
 SELECT COUNT(*) FROM song WHERE soundtrack_id = $1 AND path = $2;