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>

lib/config.ts

@@ -0,0 +1,8 @@
+/**
+ * Server-side configuration checks.
+ * Call these in route handlers or server components — never on the client.
+ */
+
+export function isTmdbConfigured(): boolean {
+  return !!process.env.TMDB_API_KEY;
+}