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;

internal/db/migrations/000007_add_uuid_and_backfill.up.sql

@@ -0,0 +1,21 @@
+-- Migration: Add UUID columns to soundtrack and song, then backfill
+
+-- Add UUID column to soundtrack (nullable for now)
+ALTER TABLE soundtrack ADD COLUMN uuid UUID NULL UNIQUE;
+
+-- Create index on uuid for performance
+CREATE INDEX IF NOT EXISTS idx_soundtrack_uuid ON soundtrack(uuid);
+
+-- Add UUID column to song (nullable for now)
+ALTER TABLE song ADD COLUMN uuid UUID NULL UNIQUE;
+
+-- Create index on uuid for performance
+CREATE INDEX IF NOT EXISTS idx_song_uuid ON song(uuid);
+
+-- Backfill existing records immediately
+UPDATE soundtrack SET uuid = gen_random_uuid() WHERE uuid IS NULL;
+UPDATE song SET uuid = gen_random_uuid() WHERE uuid IS NULL;
+
+-- Verify no nulls remain
+-- SELECT COUNT(*) FROM soundtrack WHERE uuid IS NULL;  -- Should be 0
+-- SELECT COUNT(*) FROM song WHERE uuid IS NULL;            -- Should be 0