fix(test): invalidate module-level bangumi cache between tests

The BangumiDatabase.search_all() uses a module-level TTL cache that
persists across tests using different in-memory SQLite databases.
This caused test_migrate_preserves_existing_data and test_migrate_idempotent
to return stale cached results (1 bangumi instead of 2).

Add an autouse fixture in conftest.py to clear the cache before and after
each test.

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

backend/src/test/conftest.py

@@ -8,6 +8,7 @@ from fastapi.testclient import TestClient
 from sqlmodel import Session, SQLModel, create_engine
 
 from module.api import v1
+from module.database.bangumi import _invalidate_bangumi_cache
 from module.models.config import Config
 from module.models import ResponseModel
 from module.security.api import get_current_user
@@ -18,6 +19,19 @@ from module.security.api import get_current_user
 # ---------------------------------------------------------------------------
 
 
+@pytest.fixture(autouse=True)
+def _clear_bangumi_cache():
+    """Invalidate the module-level bangumi cache before each test.
+
+    The BangumiDatabase.search_all() uses a module-level TTL cache that
+    persists across tests using different in-memory databases, causing
+    stale results.
+    """
+    _invalidate_bangumi_cache()
+    yield
+    _invalidate_bangumi_cache()
+
+
 @pytest.fixture
 def db_engine():
     """Create an in-memory SQLite engine for testing."""