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/sections/[key]/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getSection, setSection, SECTION_KEYS } from "@/lib/db";
+import { siteContent } from "@/data/content";
 import { revalidatePath } from "next/cache";
 
 type Params = { params: Promise<{ key: string }> };
@@ -10,9 +11,16 @@ export async function GET(_request: NextRequest, { params }: Params) {
     return NextResponse.json({ error: "Invalid section key" }, { status: 400 });
   }
 
-  const data = getSection(key);
+  let data = getSection(key);
   if (!data) {
-    return NextResponse.json({ error: "Section not found" }, { status: 404 });
+    // Auto-seed from fallback content if section doesn't exist yet
+    const fallback = (siteContent as unknown as Record<string, unknown>)[key];
+    if (fallback) {
+      setSection(key, fallback);
+      data = fallback;
+    } else {
+      return NextResponse.json({ error: "Section not found" }, { status: 404 });
+    }
   }
 
   return NextResponse.json(data);