feat: allow users and admins to cancel pending-approval requests

- Add cancel action to RequestActionsDropdown for admins
- Add cancel button to RequestCard for users
- Implement DELETE handler in /api/requests/[id] with:
  - Status gate: only cancellable if pending_approval or awaiting_approval
  - Clears selectedTorrent (Prisma.DbNull) on cancel
  - Fires on-grab notification job after cancel
- Tests: cancel flows for both statuses, rejection for non-cancellable status

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

src/lib/constants/notification-events.ts

@@ -77,6 +77,13 @@ export const NOTIFICATION_EVENTS = {
     severity: 'error' as const,
     priority: 'high' as const,
   },
+  request_cancelled: {
+    label: 'Request Cancelled',
+    title: 'Request Cancelled',
+    emoji: '\u{1F6AB}',
+    severity: 'warning' as const,
+    priority: 'normal' as const,
+  },
   issue_reported: {
     label: 'Issue Reported',
     title: 'Issue Reported',