feat(events): recurring expansion, CalDAV pull sync, past search, reminders

- RRULE expansion: list_events now expands recurring events into
  individual occurrences within the query window using python-dateutil
- CalDAV pull sync: new caldav_sync.py + POST /api/events/sync route;
  imports remote events into the internal store by caldav_uid
- Past event search: search_events accepts include_past=true to search
  historical events; exposed in the LLM tool definition
- Internal reminders: migration 0037 adds reminder_minutes +
  reminder_sent_at columns; event_scheduler.py checks every 5 min and
  fires push notifications; CalDAV sync job runs hourly
- reminder_minutes now stored and returned in create/update routes + tools

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

src/fabledassistant/models/event.py

@@ -27,6 +27,8 @@ class Event(Base):
     caldav_uid: Mapped[str] = mapped_column(Text, default="")
     color: Mapped[str] = mapped_column(Text, default="")
     recurrence: Mapped[str | None] = mapped_column(Text, nullable=True)
+    reminder_minutes: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    reminder_sent_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )
@@ -51,6 +53,7 @@ class Event(Base):
             "location": self.location,
             "color": self.color,
             "recurrence": self.recurrence,
+            "reminder_minutes": self.reminder_minutes,
             "created_at": self.created_at.isoformat() if self.created_at else None,
             "updated_at": self.updated_at.isoformat() if self.updated_at else None,
         }