feat: add master classes section with registration system

- New master classes section on landing page with upcoming events grid
- Admin CRUD for master classes (image, slots, trainer, style, cost, location)
- User signup modal (name + Instagram required, Telegram optional)
- Admin registration management: view, add, edit, delete with quick-contact links
- Customizable success message for signup confirmation
- Auto-filter past events, Russian date formatting, duration auto-calculation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,52 @@
+import { NextRequest, NextResponse } from "next/server";
+import { getMcRegistrations, addMcRegistration, updateMcRegistration, 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 });
+  }
+  const registrations = getMcRegistrations(title);
+  return NextResponse.json(registrations);
+}
+
+export async function POST(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { masterClassTitle, name, instagram, telegram } = body;
+    if (!masterClassTitle || !name || !instagram) {
+      return NextResponse.json({ error: "masterClassTitle, name, instagram are required" }, { status: 400 });
+    }
+    const id = addMcRegistration(masterClassTitle.trim(), name.trim(), instagram.trim(), telegram?.trim() || undefined);
+    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();
+    const { id, name, instagram, telegram } = body;
+    if (!id || !name || !instagram) {
+      return NextResponse.json({ error: "id, name, instagram are required" }, { status: 400 });
+    }
+    updateMcRegistration(id, name.trim(), instagram.trim(), telegram?.trim() || undefined);
+    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 parameter is required" }, { status: 400 });
+  }
+  const id = parseInt(idStr, 10);
+  if (isNaN(id)) {
+    return NextResponse.json({ error: "Invalid id" }, { status: 400 });
+  }
+  deleteMcRegistration(id);
+  return NextResponse.json({ ok: true });
+}