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/open-day/route.ts

@@ -0,0 +1,54 @@
+import { NextRequest, NextResponse } from "next/server";
+import {
+  getOpenDayEvents,
+  getOpenDayEvent,
+  createOpenDayEvent,
+  updateOpenDayEvent,
+  deleteOpenDayEvent,
+} from "@/lib/db";
+
+export async function GET(request: NextRequest) {
+  const idStr = request.nextUrl.searchParams.get("id");
+  if (idStr) {
+    const id = parseInt(idStr, 10);
+    if (isNaN(id)) return NextResponse.json({ error: "Invalid id" }, { status: 400 });
+    const event = getOpenDayEvent(id);
+    if (!event) return NextResponse.json({ error: "Not found" }, { status: 404 });
+    return NextResponse.json(event);
+  }
+  return NextResponse.json(getOpenDayEvents());
+}
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    if (!body.date || typeof body.date !== "string") {
+      return NextResponse.json({ error: "date is required" }, { status: 400 });
+    }
+    const id = createOpenDayEvent(body);
+    return NextResponse.json({ ok: true, id });
+  } catch {
+    return NextResponse.json({ error: "Internal error" }, { status: 500 });
+  }
+}
+
+export async function PUT(request: NextRequest) {
+  try {
+    const body = await request.json();
+    if (!body.id) return NextResponse.json({ error: "id is required" }, { status: 400 });
+    const { id, ...data } = body;
+    updateOpenDayEvent(id, data);
+    return NextResponse.json({ ok: true });
+  } catch {
+    return NextResponse.json({ error: "Internal error" }, { status: 500 });
+  }
+}
+
+export async function DELETE(request: NextRequest) {
+  const idStr = request.nextUrl.searchParams.get("id");
+  if (!idStr) return NextResponse.json({ error: "id is required" }, { status: 400 });
+  const id = parseInt(idStr, 10);
+  if (isNaN(id)) return NextResponse.json({ error: "Invalid id" }, { status: 400 });
+  deleteOpenDayEvent(id);
+  return NextResponse.json({ ok: true });
+}