fix(schedulers): wire recurring-task spawn + deliver event reminders

Drift-audit Group 2 (Phase-8 amputation — live wiring, no consumer):

- Recurring tasks never recurred: spawn_recurring_tasks() had no caller.
  Register it as a 15-min interval job in the event scheduler (which
  app.py already starts/stops). Also add a deleted_at IS NULL guard to
  the spawn query in the same change, so a trashed recurring parent can
  never resurrect children once the sweep is live.
- Event reminders were stamped reminder_sent_at but never delivered.
  _fire_reminders now creates an 'event_reminder' in-app notification
  before stamping, so a delivery failure stays retryable. Frontend
  NotificationsPanel renders the new type (⏰ + message); message logic
  pulled into a notifMessage() helper.
- Remove the dead _fire_push_notif no-op stub (push left in Phase 8) and
  its three create_task call sites — no more throwaway tasks per share.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/NotificationsPanel.vue

@@ -13,6 +13,23 @@ const typeIcon: Record<string, string> = {
   project_shared: '📁',
   note_shared: '📝',
   group_added: '👥',
+  event_reminder: '⏰',
+}
+
+function notifMessage(n: { type: string; payload: Record<string, unknown> }): string {
+  const p = n.payload
+  switch (n.type) {
+    case 'project_shared':
+      return ` shared "${p.project_title}" with you as ${p.permission}`
+    case 'note_shared':
+      return ` shared "${p.note_title}" with you as ${p.permission}`
+    case 'group_added':
+      return ` added you to "${p.group_name}" as ${p.role}`
+    case 'event_reminder':
+      return `Reminder: "${p.title}" is coming up`
+    default:
+      return ''
+  }
 }
 
 async function handleClick(notif: { id: number; payload: Record<string, unknown> }) {
@@ -49,11 +66,7 @@ onMounted(() => store.fetchAll())
         <div class="notif-body">
           <p class="notif-msg">
             <strong v-if="n.payload.invited_by">{{ n.payload.invited_by }}</strong>
-            {{ n.type === 'project_shared'
-                ? ` shared "${n.payload.project_title}" with you as ${n.payload.permission}`
-                : n.type === 'note_shared'
-                  ? ` shared "${n.payload.note_title}" with you as ${n.payload.permission}`
-                  : ` added you to "${n.payload.group_name}" as ${n.payload.role}` }}
+            {{ notifMessage(n) }}
           </p>
           <span class="notif-time">{{ relativeTime(n.created_at) }}</span>
         </div>