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>

frontend/src/views/HomeView.vue

@@ -77,9 +77,11 @@ function _dateRange() {
   const today = new Date()
   const nextWeek = new Date(today)
   nextWeek.setDate(today.getDate() + 7)
+  // Use full ISO strings so the server sees the correct UTC equivalent of
+  // local midnight / end-of-day rather than a naive UTC-midnight guess.
   return {
-    todayStr: today.toISOString().slice(0, 10) + 'T00:00:00',
-    nextWeekStr: nextWeek.toISOString().slice(0, 10) + 'T23:59:59',
+    todayStr: today.toISOString(),
+    nextWeekStr: nextWeek.toISOString(),
   }
 }
 
@@ -108,11 +110,7 @@ function _backgroundRefresh() {
 
 onMounted(async () => {
   // Phase 1: projects list + cross-project recent items + orphaned items + events — all parallel
-  const today = new Date();
-  const todayStr = today.toISOString().slice(0, 10) + "T00:00:00";
-  const nextWeek = new Date(today);
-  nextWeek.setDate(today.getDate() + 7);
-  const nextWeekStr = nextWeek.toISOString().slice(0, 10) + "T23:59:59";
+  const { todayStr, nextWeekStr } = _dateRange();
 
   const [projectsRes, recentRes, orphanTasksRes, orphanNotesRes, eventsRes] =
     await Promise.allSettled([