feat: add group duty pin notification feature

- Introduced a new configuration option `DUTY_PIN_NOTIFY` to control whether the bot re-pins the duty message when updated, providing notifications to group members.
- Updated the architecture documentation to reflect the new functionality of re-pinning duty messages.
- Enhanced the `.env.example` file to include the new configuration option with a description.
- Added tests to verify the behavior of the new refresh pin command and its integration with the existing group duty pin functionality.
- Updated internationalization messages to include help text for the new `/refresh_pin` command.

tests/test_personal_calendar_ics.py

@@ -46,7 +46,7 @@ def test_build_personal_ics_one_duty():
 
 
 def test_build_team_ics_vevent_has_description_with_full_name():
-    """Team ICS VEVENT includes DESCRIPTION with the duty holder's full_name."""
+    """Team ICS VEVENT has SUMMARY=full_name and DESCRIPTION=full_name."""
     duties = [
         _duty(1, "2025-02-20T09:00:00Z", "2025-02-20T18:00:00Z", "duty"),
     ]
@@ -56,11 +56,28 @@ def test_build_team_ics_vevent_has_description_with_full_name():
     events = [c for c in cal.walk() if c.name == "VEVENT"]
     assert len(events) == 1
     ev = events[0]
-    assert ev.get("summary") == "Duty"
+    assert ev.get("summary") == "Test User"
     assert ev.get("description") == "Test User"
     assert b"Team Calendar" in ics or "Team Calendar" in ics.decode("utf-8")
 
 
+def test_build_team_ics_empty_full_name_summary_fallback():
+    """When full_name is empty or missing, SUMMARY is 'Duty'."""
+    duty_ns = SimpleNamespace(
+        id=1,
+        start_at="2025-02-20T09:00:00Z",
+        end_at="2025-02-20T18:00:00Z",
+        event_type="duty",
+    )
+    for empty_name in ("", "   ", None):
+        duties = [(duty_ns, empty_name)]
+        ics = build_team_ics(duties)
+        cal = ICalendar.from_ical(ics)
+        events = [c for c in cal.walk() if c.name == "VEVENT"]
+        assert len(events) == 1
+        assert events[0].get("summary") == "Duty"
+
+
 def test_build_personal_ics_event_types():
     """Unavailable and vacation get correct SUMMARY."""
     duties = [