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.up.sql

@@ -0,0 +1,36 @@
+-- Migration: Add id column to song table and change PK from composite to single column
+-- This prepares the song table for eventual UUID migration
+
+-- Step 1: Add new id column (nullable initially)
+ALTER TABLE song ADD COLUMN id serial4;
+
+-- Step 2: Create unique constraint on id (allows backfilling)
+ALTER TABLE song ADD CONSTRAINT song_id_unique UNIQUE (id);
+
+-- Step 3: Backfill existing rows with sequential IDs
+-- Use DEFAULT which pulls from the sequence
+UPDATE song SET id = DEFAULT WHERE id IS NULL;
+
+-- Step 4: Verify all rows have an id
+-- If this returns 0, backfill worked
+-- SELECT COUNT(*) FROM song WHERE id IS NULL;
+
+-- Step 5: Drop the composite primary key (soundtrack_id, path)
+ALTER TABLE song DROP CONSTRAINT song_pkey;
+
+-- Step 6: Add new primary key on id column
+ALTER TABLE song ADD CONSTRAINT song_pkey PRIMARY KEY (id);
+
+-- Step 7: Ensure soundtrack_id remains a foreign key to soundtrack
+-- First drop existing FK if it exists (from the rename migration)
+ALTER TABLE song DROP CONSTRAINT IF EXISTS song_soundtrack_id_fkey;
+
+-- Then recreate it
+ALTER TABLE song ADD CONSTRAINT song_soundtrack_id_fkey 
+    FOREIGN KEY (soundtrack_id) REFERENCES soundtrack(id);
+
+-- Step 8: Create index on soundtrack_id for query performance
+CREATE INDEX IF NOT EXISTS idx_song_soundtrack_id ON song(soundtrack_id);
+
+-- Step 9: Create index on path for lookups (previously part of PK)
+CREATE INDEX IF NOT EXISTS idx_song_path ON song(path);