feat(telegram): add GET /api/telegram/channels/[channelId]/topics

Returns per-topic fetch state for a channel, mirroring the existing
account-links route auth pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/api/telegram/channels/[channelId]/topics/route.ts

@@ -0,0 +1,17 @@
+import { NextResponse } from "next/server";
+import { authenticateApiRequest } from "@/lib/telegram/api-auth";
+import { listChannelTopics } from "@/lib/telegram/admin-queries";
+
+export const dynamic = "force-dynamic";
+
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ channelId: string }> }
+) {
+  const authResult = await authenticateApiRequest(request, true);
+  if ("error" in authResult) return authResult.error;
+
+  const { channelId } = await params;
+  const topics = await listChannelTopics(channelId);
+  return NextResponse.json(topics);
+}