fix(events): audit pass — 7 correctness fixes across the events system

Backend:
- tools.py: apply UTC normalization to update_event datetime fields
  (matched create_event which already did this)
- events.py service: allow end_dt/recurrence/project_id to be cleared
  via update_event by permitting None for nullable fields
- events.py service: find_events_by_query now returns upcoming events
  first, falling back to past — prevents AI tools from mutating stale
  past events when a future match exists
- events.py service: list_events now uses overlap logic (start <= to
  AND end >= from) so multi-day events spanning the query boundary
  are included; previously only start_dt was checked

Frontend:
- ToolCallCard: fire fable:calendar-changed on created/updated/deleted
  so CalendarView refetches without requiring a manual page refresh
- KnowledgeView: replace raw apiGet('/api/events') with listEvents()
  client function; also fix today bar which was reading .events off a
  flat array (always empty) — now correctly receives EventEntry[]
- HomeView: use full ISO strings for event date range instead of naive
  UTC-midnight strings; deduplicate inline date math via _dateRange()

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

src/fabledassistant/routes/events.py

@@ -96,11 +96,15 @@ async def update_event(event_id: int):
         if int_field in data:
             fields[int_field] = data[int_field]
     for dt_field in ("start_dt", "end_dt"):
-        if dt_field in data and data[dt_field]:
-            try:
-                fields[dt_field] = _parse_dt(data[dt_field])
-            except ValueError:
-                return jsonify({"error": f"Invalid datetime for {dt_field}"}), 400
+        if dt_field in data:
+            if data[dt_field] is None:
+                # Explicit null clears the field (e.g. removing end_dt)
+                fields[dt_field] = None
+            elif data[dt_field]:
+                try:
+                    fields[dt_field] = _parse_dt(data[dt_field])
+                except ValueError:
+                    return jsonify({"error": f"Invalid datetime for {dt_field}"}), 400
     event = await events_svc.update_event(
         user_id=_get_current_user_id(),
         event_id=event_id,