feat: add calendar subscription token functionality and ICS generation

- Introduced a new database model for calendar subscription tokens, allowing users to generate unique tokens for accessing their personal calendar.
- Implemented API endpoint to return ICS files containing only the subscribing user's duties, enhancing user experience with personalized calendar access.
- Added utility functions for generating ICS files from user duties, ensuring proper formatting and timezone handling.
- Updated command handlers to support the new calendar link feature, providing users with easy access to their personal calendar subscriptions.
- Included unit tests for the new functionality, ensuring reliability and correctness of token generation and ICS file creation.

tests/test_app.py

@@ -248,3 +248,50 @@ def test_duties_200_with_unknown_event_type_mapped_to_duty(client):
     assert len(data) == 1
     assert data[0]["event_type"] == "duty"
     assert data[0]["full_name"] == "User A"
+
+
+@patch("duty_teller.api.app.get_user_by_calendar_token")
+def test_calendar_ical_404_unknown_token(mock_get_user, client):
+    """GET /api/calendar/ical/{token}.ics with unknown token returns 404."""
+    mock_get_user.return_value = None
+    r = client.get("/api/calendar/ical/unknown-token-xyz.ics")
+    assert r.status_code == 404
+    assert "not found" in r.text.lower()
+    mock_get_user.assert_called_once()
+
+
+@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_200_returns_only_that_users_duties(
+    mock_get_user, mock_get_duties, mock_build_ics, client
+):
+    """GET /api/calendar/ical/{token}.ics returns ICS with only the token owner's duties."""
+    from types import SimpleNamespace
+
+    mock_user = SimpleNamespace(id=1, full_name="User A")
+    mock_get_user.return_value = mock_user
+    duty = SimpleNamespace(
+        id=10,
+        user_id=1,
+        start_at="2026-06-15T09:00:00Z",
+        end_at="2026-06-15T18:00:00Z",
+        event_type="duty",
+    )
+    mock_get_duties.return_value = [(duty, "User A")]
+    mock_build_ics.return_value = (
+        b"BEGIN:VCALENDAR\r\nVEVENT\r\n2026-06-15\r\nEND:VCALENDAR"
+    )
+
+    r = client.get("/api/calendar/ical/valid-token.ics")
+    assert r.status_code == 200
+    assert r.headers.get("content-type", "").startswith("text/calendar")
+    assert b"BEGIN:VCALENDAR" in r.content
+    mock_get_user.assert_called_once()
+    mock_get_duties.assert_called_once_with(ANY, 1, from_date=ANY, to_date=ANY)
+    mock_build_ics.assert_called_once()
+    # Only User A's duty was passed to build_personal_ics
+    duties_arg = mock_build_ics.call_args[0][0]
+    assert len(duties_arg) == 1
+    assert duties_arg[0][0].user_id == 1
+    assert duties_arg[0][1] == "User A"