feat: admin notification system with priority ordering, bell icon, and /notifications page

Two-tier notification system: new_booking (all public bookings) + pending_booking (notes/today).
Priority-sorted queue, unread count polling, enriched responses with user_name/booking_start_time.
Fix critical bug: edit_requested cleanup was broken (wrong reason string in 3 handlers).
Add 15 new tests covering priority ordering, enrichment, and notification creation flows.
Update Admin Manual, Technical Manual, and gap backlog docs.

backend/handlers/user/account.go

@@ -45,6 +45,7 @@ func DeleteAccountHandler(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "server error", http.StatusInternalServerError)
 			return
 		}
+		// TODO: Create 'user_anonymized' notification for admin audit trail
 	}
 
 	// Delete CardDAV contact (non-blocking, best-effort)