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/search/route.ts

@@ -1,8 +1,19 @@
 import { type NextRequest, NextResponse } from "next/server";
+import { isTmdbConfigured } from "@/lib/config";
 import { searchMovies, searchMulti, searchTv } from "@/lib/tmdb/client";
 import type { TmdbSearchResponse } from "@/lib/tmdb/types";
 
 export async function GET(req: NextRequest) {
+  if (!isTmdbConfigured()) {
+    return NextResponse.json(
+      {
+        error: "TMDB API key is not configured. Visit /setup for instructions.",
+        code: "TMDB_NOT_CONFIGURED",
+      },
+      { status: 503 },
+    );
+  }
+
   const query = req.nextUrl.searchParams.get("query");
   const type = req.nextUrl.searchParams.get("type");