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/app/api/admin/mc-registrations/route.ts

@@ -1,13 +1,13 @@
 import { NextRequest, NextResponse } from "next/server";
-import { getMcRegistrations, addMcRegistration, updateMcRegistration, deleteMcRegistration } from "@/lib/db";
+import { getMcRegistrations, getAllMcRegistrations, addMcRegistration, updateMcRegistration, toggleMcNotification, deleteMcRegistration } from "@/lib/db";
 
 export async function GET(request: NextRequest) {
   const title = request.nextUrl.searchParams.get("title");
-  if (!title) {
-    return NextResponse.json({ error: "title parameter is required" }, { status: 400 });
+  if (title) {
+    return NextResponse.json(getMcRegistrations(title));
   }
-  const registrations = getMcRegistrations(title);
-  return NextResponse.json(registrations);
+  // No title = return all registrations
+  return NextResponse.json(getAllMcRegistrations());
 }
 
 export async function POST(request: NextRequest) {
@@ -27,6 +27,21 @@ export async function POST(request: NextRequest) {
 export async function PUT(request: NextRequest) {
   try {
     const body = await request.json();
+
+    // Toggle notification status
+    if (body.action === "toggle-notify") {
+      const { id, field, value } = body;
+      if (!id || !field || typeof value !== "boolean") {
+        return NextResponse.json({ error: "id, field, value are required" }, { status: 400 });
+      }
+      if (field !== "notified_confirm" && field !== "notified_reminder") {
+        return NextResponse.json({ error: "Invalid field" }, { status: 400 });
+      }
+      toggleMcNotification(id, field, value);
+      return NextResponse.json({ ok: true });
+    }
+
+    // Regular update
     const { id, name, instagram, telegram } = body;
     if (!id || !name || !instagram) {
       return NextResponse.json({ error: "id, name, instagram are required" }, { status: 400 });