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/group-booking/route.ts

@@ -0,0 +1,24 @@
+import { NextRequest, NextResponse } from "next/server";
+import { addGroupBooking } from "@/lib/db";
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { name, phone, groupInfo, instagram, telegram } = body;
+
+    if (!name || typeof name !== "string" || !phone || typeof phone !== "string") {
+      return NextResponse.json({ error: "name and phone are required" }, { status: 400 });
+    }
+
+    const cleanName = name.trim().slice(0, 100);
+    const cleanPhone = phone.trim().slice(0, 30);
+    const cleanGroup = typeof groupInfo === "string" ? groupInfo.trim().slice(0, 200) : undefined;
+    const cleanIg = typeof instagram === "string" ? instagram.trim().slice(0, 100) : undefined;
+    const cleanTg = typeof telegram === "string" ? telegram.trim().slice(0, 100) : undefined;
+
+    const id = addGroupBooking(cleanName, cleanPhone, cleanGroup, cleanIg, cleanTg);
+    return NextResponse.json({ ok: true, id });
+  } catch {
+    return NextResponse.json({ error: "Internal error" }, { status: 500 });
+  }
+}