feat: saved searches (bookmarks)

- Add saved_searches_collection to database.py with index on created_by+created_at
- New routes/saved_searches.py: GET /api/saved-searches, POST, DELETE
- Saved searches are scoped per user (created_by = token sub)
- Mount router in main.py
- Frontend: Save filters button, saved search pills with load/delete
- loadSavedSearches called on initApp
- applySavedSearch restores filters and validates services against current options
- Add CSS for saved-searches row
- Add tests for CRUD, delete 404, and name validation

backend/tests/conftest.py

@@ -22,9 +22,11 @@ def mock_watermarks_collection():
 @pytest.fixture(scope="function")
 def client(mock_events_collection, mock_watermarks_collection, monkeypatch):
     monkeypatch.setattr("database.events_collection", mock_events_collection)
+    monkeypatch.setattr("database.saved_searches_collection", mock_events_collection)
     monkeypatch.setattr("routes.fetch.events_collection", mock_events_collection)
     monkeypatch.setattr("routes.events.events_collection", mock_events_collection)
     monkeypatch.setattr("routes.ask.events_collection", mock_events_collection)
+    monkeypatch.setattr("routes.saved_searches.saved_searches_collection", mock_events_collection)
     monkeypatch.setattr("watermark.watermarks_collection", mock_watermarks_collection)
     monkeypatch.setattr("routes.health.watermarks_collection", mock_watermarks_collection)
     monkeypatch.setattr("routes.fetch.get_watermark", lambda source: None)