fix(tests): drop_all before create_all to clear stale PostgreSQL enums

If a previous test run was interrupted before teardown, PostgreSQL enum
types persist with stale values, causing create_all to fail. Adding
drop_all first ensures a clean slate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -24,6 +24,7 @@ async def engine():
     """Create the test engine and schema once for the entire session."""
     eng = create_async_engine(TEST_DATABASE_URL, echo=False)
     async with eng.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
         await conn.run_sync(Base.metadata.create_all)
     yield eng
     async with eng.begin() as conn: