fix: isolate tests on in-memory SQLite to protect production database

Tests were importing the production engine and dropping all tables on
teardown, corrupting topology.db in the Docker volume. Set DATABASE_URL
to sqlite:///:memory: before any import in the test file, and use
StaticPool in database.py when running against :memory: so all
connections share the same in-memory database.

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

backend/database.py

@@ -9,10 +9,12 @@ os.makedirs("data", exist_ok=True)
 
 _DATABASE_URL = os.environ.get("DATABASE_URL", "sqlite:///./data/topology.db")
 
-engine = create_engine(
-    _DATABASE_URL,
-    connect_args={"check_same_thread": False}
-)
+_engine_kwargs: dict = {"connect_args": {"check_same_thread": False}}
+if ":memory:" in _DATABASE_URL:
+    from sqlalchemy.pool import StaticPool
+    _engine_kwargs["poolclass"] = StaticPool
+
+engine = create_engine(_DATABASE_URL, **_engine_kwargs)
 
 
 @event.listens_for(Engine, "connect")