test: tidy conftest fixtures per review (drop dead try/finally, scope override cleanup, rm unused import)

tests/conftest.py

@@ -1,6 +1,5 @@
 """Test harness: a throwaway SQLite DB per test, get_db overridden, a TestClient
 that does NOT run lifespan startup (so schedulers/SLMM polling stay off)."""
-import os
 import uuid
 import pytest
 from datetime import datetime
@@ -30,15 +29,12 @@ def db_session(tmp_path):
 def client(db_session):
     from backend.main import app  # imported lazily so module side effects are contained
     def _override():
-        try:
         yield db_session
-        finally:
-            pass
     app.dependency_overrides[get_db] = _override
     # No `with` → lifespan/startup events do not run (no scheduler/SLMM threads).
     c = TestClient(app)
     yield c
-    app.dependency_overrides.clear()
+    app.dependency_overrides.pop(get_db, None)
 
 
 @pytest.fixture(autouse=True)