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_personal_calendar_ics.py

@@ -0,0 +1,60 @@
+"""Unit tests for personal calendar ICS generation."""
+
+from types import SimpleNamespace
+
+from icalendar import Calendar as ICalendar
+
+from duty_teller.api.personal_calendar_ics import build_personal_ics
+
+
+# Minimal Duty-like object for build_personal_ics(duties_with_name: list[tuple[Duty, str]])
+def _duty(id_, start_at, end_at, event_type="duty"):
+    return (
+        SimpleNamespace(
+            id=id_, start_at=start_at, end_at=end_at, event_type=event_type
+        ),
+        "Test User",
+    )
+
+
+def test_build_personal_ics_empty_returns_valid_calendar():
+    """Empty list produces VCALENDAR with no VEVENTs."""
+    ics = build_personal_ics([])
+    assert ics.startswith(b"BEGIN:VCALENDAR")
+    assert ics.rstrip().endswith(b"END:VCALENDAR")
+    cal = ICalendar.from_ical(ics)
+    assert cal is not None
+    events = [c for c in cal.walk() if c.name == "VEVENT"]
+    assert len(events) == 0
+
+
+def test_build_personal_ics_one_duty():
+    """One duty produces one VEVENT with correct DTSTART/DTEND/SUMMARY."""
+    duties = [
+        _duty(1, "2025-02-20T09:00:00Z", "2025-02-20T18:00:00Z", "duty"),
+    ]
+    ics = build_personal_ics(duties)
+    cal = ICalendar.from_ical(ics)
+    assert cal is not None
+    events = [c for c in cal.walk() if c.name == "VEVENT"]
+    assert len(events) == 1
+    ev = events[0]
+    assert ev.get("summary") == "Duty"
+    assert "2025-02-20" in str(ev.get("dtstart").dt)
+    assert "2025-02-20" in str(ev.get("dtend").dt)
+    assert b"duty-1@duty-teller" in ics or "duty-1@duty-teller" in ics.decode("utf-8")
+
+
+def test_build_personal_ics_event_types():
+    """Unavailable and vacation get correct SUMMARY."""
+    duties = [
+        _duty(1, "2025-02-20T09:00:00Z", "2025-02-21T09:00:00Z", "unavailable"),
+        _duty(2, "2025-02-25T00:00:00Z", "2025-03-01T00:00:00Z", "vacation"),
+    ]
+    ics = build_personal_ics(duties)
+    cal = ICalendar.from_ical(ics)
+    events = [c for c in cal.walk() if c.name == "VEVENT"]
+    assert len(events) == 2
+    summaries = [str(ev.get("summary")) for ev in events]
+    assert "Unavailable" in summaries
+    assert "Vacation" in summaries