Rename TMDB_API_KEY to TMDB_API_READ_ACCESS_TOKEN and add optional base URL overrides

Renames the env var for clarity since it holds a read access token, not an API
key. Adds optional TMDB_API_BASE_URL and TMDB_IMAGE_BASE_URL env vars for
advanced users. Centralizes image URL construction into a shared tmdbImageUrl()
helper, replacing hardcoded URLs across all components.

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

lib/tmdb/image.ts

@@ -0,0 +1,7 @@
+const IMAGE_BASE_URL =
+  process.env.TMDB_IMAGE_BASE_URL || "https://image.tmdb.org/t/p";
+
+export function tmdbImageUrl(path: string | null, size = "w500") {
+  if (!path) return null;
+  return `${IMAGE_BASE_URL}/${size}${path}`;
+}