Add GitHub metadata enrichment for catalog apps

Enrich catalog apps with GitHub API data (stars, version, downloads,
release date) via two strategies: background drip for repo-level info
and on-demand fetch when opening a detail page.

- Add github_enrichment module with API calls, asset filtering, and
  architecture auto-detection for AppImage downloads
- DB migrations v14/v15 for GitHub metadata and release asset columns
- Extract github_owner/repo from feed links during catalog sync
- Display colored stat cards (stars, version, downloads, released) on
  detail pages with on-demand enrichment
- Show stars and version on browse tiles and featured carousel cards
- Replace install button with SplitButton dropdown when multiple arch
  assets available, preferring detected architecture
- Disable install button until enrichment completes to prevent stale
  AppImageHub URL downloads
- Keep enrichment banner visible on catalog page until truly complete,
  showing paused state when rate-limited
- Add GitHub token and auto-enrich toggle to preferences

data/app.driftwood.Driftwood.gschema.xml

@@ -142,5 +142,15 @@
       <summary>Watch removable media</summary>
       <description>Scan removable drives for AppImages when mounted.</description>
     </key>
+    <key name="github-token" type="s">
+      <default>''</default>
+      <summary>GitHub personal access token</summary>
+      <description>Optional GitHub token for higher API rate limits (5,000 vs 60 requests per hour).</description>
+    </key>
+    <key name="catalog-auto-enrich" type="b">
+      <default>true</default>
+      <summary>Auto-enrich catalog apps</summary>
+      <description>Automatically fetch GitHub metadata (stars, version, downloads) for catalog apps in the background.</description>
+    </key>
   </schema>
 </schemalist>