refactor: simplify ICS calendar API to return only duty shifts

- Removed the ability to specify multiple event types in the ICS calendar generation API, ensuring it only returns duty shifts.
- Updated the associated test to reflect the change in behavior, confirming that unknown query parameters are ignored.
- Revised documentation to clarify the API's focus on duty shifts only, enhancing clarity for users.

tests/test_app.py

@@ -336,10 +336,10 @@ def test_calendar_ical_200_returns_only_that_users_duties(
 @patch("duty_teller.api.app.build_personal_ics")
 @patch("duty_teller.api.app.get_duties_for_user")
 @patch("duty_teller.api.app.get_user_by_calendar_token")
-def test_calendar_ical_events_all_returns_all_event_types(
+def test_calendar_ical_ignores_unknown_query_params(
     mock_get_user, mock_get_duties, mock_build_ics, client
 ):
-    """GET /api/calendar/ical/{token}.ics?events=all returns ICS with duty, unavailable, vacation."""
+    """Unknown query params (e.g. events=all) are ignored; response is duty-only."""
     from types import SimpleNamespace
 
     mock_user = SimpleNamespace(id=1, full_name="User A")
@@ -351,39 +351,15 @@ def test_calendar_ical_events_all_returns_all_event_types(
         end_at="2026-06-15T18:00:00Z",
         event_type="duty",
     )
-    unavailable = SimpleNamespace(
-        id=11,
-        user_id=1,
-        start_at="2026-06-16T09:00:00Z",
-        end_at="2026-06-16T18:00:00Z",
-        event_type="unavailable",
-    )
-    vacation = SimpleNamespace(
-        id=12,
-        user_id=1,
-        start_at="2026-06-17T09:00:00Z",
-        end_at="2026-06-17T18:00:00Z",
-        event_type="vacation",
-    )
-    mock_get_duties.return_value = [
-        (duty, "User A"),
-        (unavailable, "User A"),
-        (vacation, "User A"),
-    ]
-    mock_build_ics.return_value = b"BEGIN:VCALENDAR\r\nVEVENT\r\nEND:VCALENDAR"
-    token = "y" * 43
+    mock_get_duties.return_value = [(duty, "User A")]
+    mock_build_ics.return_value = b"BEGIN:VCALENDAR\r\nEND:VCALENDAR"
+    token = "z" * 43
 
     r = client.get(f"/api/calendar/ical/{token}.ics", params={"events": "all"})
     assert r.status_code == 200
-    assert r.headers.get("content-type", "").startswith("text/calendar")
     mock_get_duties.assert_called_once_with(
-        ANY, 1, from_date=ANY, to_date=ANY, event_types=None
+        ANY, 1, from_date=ANY, to_date=ANY, event_types=["duty"]
     )
-    duties_arg = mock_build_ics.call_args[0][0]
-    assert len(duties_arg) == 3
-    assert duties_arg[0][0].event_type == "duty"
-    assert duties_arg[1][0].event_type == "unavailable"
-    assert duties_arg[2][0].event_type == "vacation"
 
 
 # --- /api/calendar-events ---