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/migrations/000007_add_uuid_and_backfill.down.sql

@@ -0,0 +1,9 @@
+-- Rollback: Remove UUID columns from soundtrack and song tables
+
+-- Drop indexes
+DROP INDEX IF EXISTS idx_soundtrack_uuid;
+DROP INDEX IF EXISTS idx_song_uuid;
+
+-- Drop UUID columns
+ALTER TABLE soundtrack DROP COLUMN IF EXISTS uuid;
+ALTER TABLE song DROP COLUMN IF EXISTS uuid;