Document task DB access and unify background task DB handling

backend/tests/test_tasks/test_geo_cache_flush.py

@@ -34,6 +34,7 @@ def _make_app(flush_count: int = 0) -> MagicMock:
     app.state.db.close = AsyncMock()
     app.state.scheduler = MagicMock()
     app.state.settings = MagicMock(database_path="/tmp/fake.db")
+    app.state.runtime_settings = None
     return app
 
 
@@ -51,7 +52,7 @@ class TestRunFlush:
         app = _make_app()
 
         with patch(
-            "app.tasks.geo_cache_flush.open_db",
+            "app.tasks.db.open_db",
             new_callable=AsyncMock,
             return_value=app.state.db,
         ), patch(
@@ -69,7 +70,7 @@ class TestRunFlush:
         app = _make_app()
 
         with patch(
-            "app.tasks.geo_cache_flush.open_db",
+            "app.tasks.db.open_db",
             new_callable=AsyncMock,
             return_value=app.state.db,
         ), patch(
@@ -89,7 +90,7 @@ class TestRunFlush:
         app = _make_app()
 
         with patch(
-            "app.tasks.geo_cache_flush.open_db",
+            "app.tasks.db.open_db",
             new_callable=AsyncMock,
             return_value=app.state.db,
         ), patch(