Add attend_start/attend_end fields for partial conference attendance.

Allows recording when you arrive late or leave early at a multi-day
conference. Both fields accept a plain date or datetime with time.
Trip pages display the attendance dates instead of the official
conference dates when these fields are set.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agenda/conference.py

@@ -36,6 +36,8 @@ class Conference:
     currency: str | None = None
     latitude: float | None = None
     longitude: float | None = None
+    attend_start: date | datetime | None = None
+    attend_end: date | datetime | None = None
     cfp_end: date | None = None
     cfp_url: str | None = None
     free: bool | None = None