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/KnowledgeView.vue

@@ -1,7 +1,7 @@
 <script setup lang="ts">
 import { ref, computed, watch, onMounted, onUnmounted, nextTick } from "vue";
 import { useRouter } from "vue-router";
-import { apiGet, apiPatch } from "@/api/client";
+import { apiGet, apiPatch, listEvents } from "@/api/client";
 import { fmtCompact } from "@/utils/dateFormat";
 import { useChatStore } from "@/stores/chat";
 import GraphView from "@/views/GraphView.vue";
@@ -167,15 +167,13 @@ async function fetchTodayBar() {
   try {
     const now = new Date();
     const end = new Date(now.getTime() + 7 * 86_400_000);
-    const [evData, taskData] = await Promise.all([
-      apiGet<{ events: UpcomingEvent[] }>(
-        `/api/events?from=${now.toISOString().slice(0, 10)}&to=${end.toISOString().slice(0, 10)}`
-      ).catch(() => ({ events: [] as UpcomingEvent[] })),
+    const [events, taskData] = await Promise.all([
+      listEvents(now.toISOString(), end.toISOString()).catch(() => [] as UpcomingEvent[]),
       apiGet<{ total: number }>(
         `/api/tasks?status=todo&status=in_progress&overdue=true&limit=1`
       ).catch(() => ({ total: 0 })),
     ]);
-    upcomingEvents.value = (evData.events ?? []).slice(0, 3);
+    upcomingEvents.value = events.slice(0, 3);
     overdueCount.value = taskData.total ?? 0;
   } catch { /* silent */ }
 }