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/open-day-register/route.ts

@@ -0,0 +1,54 @@
+import { NextRequest, NextResponse } from "next/server";
+import {
+  addOpenDayBooking,
+  isOpenDayClassBookedByPhone,
+  getPersonOpenDayBookings,
+  getOpenDayEvent,
+} from "@/lib/db";
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { classId, eventId, name, phone, instagram, telegram } = body;
+
+    if (!classId || !eventId || !name || !phone) {
+      return NextResponse.json({ error: "classId, eventId, name, phone are required" }, { status: 400 });
+    }
+
+    const cleanName = (name as string).trim().slice(0, 100);
+    const cleanPhone = (phone as string).replace(/\D/g, "").slice(0, 15);
+    const cleanIg = instagram ? (instagram as string).trim().slice(0, 100) : undefined;
+    const cleanTg = telegram ? (telegram as string).trim().slice(0, 100) : undefined;
+
+    if (!cleanPhone) {
+      return NextResponse.json({ error: "Invalid phone" }, { status: 400 });
+    }
+
+    // Check not already booked
+    if (isOpenDayClassBookedByPhone(classId, cleanPhone)) {
+      return NextResponse.json({ error: "Вы уже записаны на это занятие" }, { status: 409 });
+    }
+
+    const id = addOpenDayBooking(classId, eventId, {
+      name: cleanName,
+      phone: cleanPhone,
+      instagram: cleanIg,
+      telegram: cleanTg,
+    });
+
+    // Return total bookings for this person (for discount calculation)
+    const totalBookings = getPersonOpenDayBookings(eventId, cleanPhone);
+    const event = getOpenDayEvent(eventId);
+    const pricePerClass = event && totalBookings >= event.discountThreshold
+      ? event.discountPrice
+      : event?.pricePerClass ?? 30;
+
+    return NextResponse.json({ ok: true, id, totalBookings, pricePerClass });
+  } catch (e) {
+    const msg = e instanceof Error ? e.message : "Internal error";
+    if (msg.includes("UNIQUE")) {
+      return NextResponse.json({ error: "Вы уже записаны на это занятие" }, { status: 409 });
+    }
+    return NextResponse.json({ error: msg }, { status: 500 });
+  }
+}