Add setup page and graceful handling for missing TMDB API key

Without TMDB_API_KEY the app silently fails on search and import.
This adds a /setup page that guides admins through obtaining and
configuring the key, redirects unconfigured visitors from the
landing page, and returns clear error messages from the search API.

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

app/api/setup/status/route.ts

@@ -0,0 +1,8 @@
+import { NextResponse } from "next/server";
+import { isTmdbConfigured } from "@/lib/config";
+
+export async function GET() {
+  return NextResponse.json({
+    tmdbConfigured: isTmdbConfigured(),
+  });
+}