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/MasterClasses.tsx

@@ -5,7 +5,7 @@ import Image from "next/image";
 import { Calendar, Clock, User, MapPin, Instagram } from "lucide-react";
 import { SectionHeading } from "@/components/ui/SectionHeading";
 import { Reveal } from "@/components/ui/Reveal";
-import { MasterClassSignupModal } from "@/components/ui/MasterClassSignupModal";
+import { SignupModal } from "@/components/ui/SignupModal";
 import type { SiteContent, MasterClassItem, MasterClassSlot } from "@/types";
 
 interface MasterClassesProps {
@@ -236,10 +236,12 @@ export function MasterClasses({ data }: MasterClassesProps) {
         )}
       </div>
 
-      <MasterClassSignupModal
+      <SignupModal
         open={signupTitle !== null}
         onClose={() => setSignupTitle(null)}
-        masterClassTitle={signupTitle ?? ""}
+        subtitle={signupTitle ?? ""}
+        endpoint="/api/master-class-register"
+        extraBody={{ masterClassTitle: signupTitle }}
         successMessage={data.successMessage}
       />
     </section>