feat: Add id column to song table and prep for UUID migration

- Add id serial4 PK to song table (was composite PK)
- Update queries to use soundtrack_id + path
- Add UUID columns to soundtrack and song (nullable)
- Add migration tracking table

TODO: Run sqlc generate, then create backfill migration (000008)

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

internal/db/migrations/000006_add_id_to_song.down.sql

@@ -0,0 +1,24 @@
+-- Rollback: Remove id column and restore composite PK
+
+-- Step 1: Drop indexes created in up migration
+DROP INDEX IF EXISTS idx_song_soundtrack_id;
+DROP INDEX IF EXISTS idx_song_path;
+
+-- Step 2: Drop foreign key constraint
+ALTER TABLE song DROP CONSTRAINT IF EXISTS song_soundtrack_id_fkey;
+
+-- Step 3: Drop new primary key
+ALTER TABLE song DROP CONSTRAINT song_pkey;
+
+-- Step 4: Drop unique constraint on id
+ALTER TABLE song DROP CONSTRAINT IF EXISTS song_id_unique;
+
+-- Step 5: Restore composite primary key
+ALTER TABLE song ADD CONSTRAINT song_pkey PRIMARY KEY (soundtrack_id, path);
+
+-- Step 6: Drop the id column
+ALTER TABLE song DROP COLUMN id;
+
+-- Step 7: Recreate original foreign key (soundtrack_id references soundtrack.id)
+ALTER TABLE song ADD CONSTRAINT song_soundtrack_id_fkey 
+    FOREIGN KEY (soundtrack_id) REFERENCES soundtrack(id);