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/sections/team/TeamProfile.tsx

@@ -2,7 +2,7 @@ import { useState, useEffect, useRef, useCallback } from "react";
 import Image from "next/image";
 import { ArrowLeft, Instagram, Trophy, GraduationCap, ExternalLink, X, Award, Scale, Clock, MapPin } from "lucide-react";
 import type { TeamMember, RichListItem, VictoryItem, ScheduleLocation } from "@/types/content";
-import { BookingModal } from "@/components/ui/BookingModal";
+import { SignupModal } from "@/components/ui/SignupModal";
 
 interface TeamProfileProps {
   member: TeamMember;
@@ -329,10 +329,12 @@ export function TeamProfile({ member, onBack, schedule }: TeamProfileProps) {
         </div>
       )}
 
-      <BookingModal
+      <SignupModal
         open={bookingGroup !== null}
         onClose={() => setBookingGroup(null)}
-        groupInfo={bookingGroup ?? undefined}
+        subtitle={bookingGroup ?? undefined}
+        endpoint="/api/group-booking"
+        extraBody={{ groupInfo: bookingGroup }}
       />
     </div>
   );