Fix Python SyntaxWarning in database migration regex

Use raw string (r"") for SQL containing regex pattern to avoid
Python SyntaxWarning about invalid escape sequence '\.'

backend/app/database.py

@@ -210,7 +210,7 @@ def _run_migrations():
         END $$;
         """,
         # Migrate existing semver tags to package_versions
-        """
+        r"""
         DO $$
         BEGIN
             IF EXISTS (SELECT 1 FROM information_schema.tables WHERE table_name = 'package_versions') THEN