feat: add booking management, Open Day, unified signup modal

- MC registrations: notification toggles (confirm/remind) with urgency
- Group bookings: save to DB from BookingModal, admin CRUD at /admin/bookings
- Open Day: full event system with schedule grid (halls × time), per-class
  booking, discount pricing (30 BYN / 20 BYN from 3+), auto-cancel threshold
- Unified SignupModal replaces 3 separate forms — consistent fields
  (name, phone, instagram, telegram), Instagram DM fallback on network error
- Centralized /admin/bookings page with 3 tabs (classes, MC, Open Day),
  collapsible sections, notification toggles, filter chips
- Unread booking badge on sidebar + dashboard widget with per-type breakdown
- Pricing: contact hint (Instagram/Telegram/phone) on price & rental tabs,
  admin toggle to show/hide
- DB migrations 5-7: group_bookings table, open_day tables, unified fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/ui/BookingModal.tsx

@@ -71,6 +71,12 @@ export function BookingModal({ open, onClose, groupInfo, contact: contactProp }:
   const handleSubmit = useCallback(
     (e: React.FormEvent) => {
       e.preventDefault();
+      // Save booking to DB (fire-and-forget)
+      fetch("/api/group-booking", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ name, phone, groupInfo }),
+      }).catch(() => {});
       // Build Instagram DM message with pre-filled text
       const groupText = groupInfo ? ` (${groupInfo})` : "";
       const message = `Здравствуйте! Меня зовут ${name}, хочу записаться на занятие${groupText}. Мой телефон: ${phone}`;