feat(website): add the TMDB metadata guide with the required attribution

New guide at /blog/tmdb-metadata-guide/: a sent/not-sent table for the
opt-in enrichment (title, year, app language and the build's API key leave
the machine; nothing about the user, the provider or the playlist does),
how to switch it on, when to use your own free key, what the feature
unlocks, and what the local cache holds. Eight FAQ entries, and a section
for readers who would rather leave it off.

The post states plainly that TMDB is a metadata database and not a content
source, and carries TMDB's required attribution through a reusable
TmdbAttribution component (their logo plus "This product uses the TMDB API
but is not endorsed or certified by TMDB."), the same wording the app shows
in Settings and About.

Its screenshot is the settings section itself: the capture's G5 guard keeps
enrichment disabled for the whole run, so no licensed poster or still can
reach a published frame. The new action stages the switch in the form only,
which reveals the key field, the cache panel and the attribution while the
stored setting stays off.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/testing/website-guides.test.mjs

@@ -65,6 +65,10 @@ const GUIDES = [
       'blog/guides/screenshots/guide-epg-offset-dark.png',
     ],
   },
+  {
+    slug: 'tmdb-metadata-guide',
+    screenshots: ['blog/guides/screenshots/guide-tmdb-settings-dark.png'],
+  },
 ];
 
 const readDist = (relativePath) => readFile(new URL(relativePath, distRoot), 'utf8');