Make MyPy happy

Fixes for various type errors throw by mypy.

portal/api/agenda.py

@@ -7,7 +7,7 @@ from ..model import Key
 
 @bp.get("/agendas/-/items")
 @require_valid(Key)
-def the_agenda_items():
+def the_agenda_items() -> list[dict[str, str]]:
     tasks = sorted(
         TaigaUserStoryProvider.get_the_one().fetch_user_stories(),
         key=lambda task: task["kanban_order"],