feat: add group preview image API endpoint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/api/groups/[id]/preview/route.ts

@@ -0,0 +1,36 @@
+import { NextResponse } from "next/server";
+import { prisma } from "@/lib/prisma";
+import { authenticateApiRequest } from "@/lib/telegram/api-auth";
+
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const authResult = await authenticateApiRequest(request);
+  if ("error" in authResult) return authResult.error;
+
+  const { id } = await params;
+
+  const group = await prisma.packageGroup.findUnique({
+    where: { id },
+    select: { previewData: true },
+  });
+
+  if (!group || !group.previewData) {
+    return new NextResponse(null, { status: 404 });
+  }
+
+  const buffer =
+    group.previewData instanceof Buffer
+      ? group.previewData
+      : Buffer.from(group.previewData);
+
+  return new NextResponse(buffer, {
+    status: 200,
+    headers: {
+      "Content-Type": "image/jpeg",
+      "Content-Length": String(buffer.length),
+      "Cache-Control": "public, max-age=3600, immutable",
+    },
+  });
+}