fix(backend): solve 500 errors with auto-migrations and standardized filenames

migrations/004_add_user_roles.up.sql

@@ -0,0 +1,5 @@
+-- Add role column to users table
+ALTER TABLE users ADD COLUMN role VARCHAR(20) NOT NULL DEFAULT 'user';
+
+-- Set first user as admin (if any)
+UPDATE users SET role = 'admin' WHERE id = (SELECT id FROM users ORDER BY created_at ASC LIMIT 1);