feat: add metadata statistics to dashboard

Add a new metadata row to the dashboard with three cards:
- Series metadata coverage (linked vs unlinked donut)
- Provider breakdown (donut by provider)
- Book metadata quality (summary and ISBN fill rates)

Includes API changes (stats.rs), frontend types, and FR/EN translations.

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

apps/backoffice/lib/i18n/fr.ts

@@ -73,6 +73,13 @@ const fr = {
   "dashboard.noSeries": "Aucune série pour le moment",
   "dashboard.unknown": "Inconnu",
   "dashboard.readCount": "{{read}}/{{total}} lu",
+  "dashboard.metadataCoverage": "Couverture métadonnées",
+  "dashboard.seriesLinked": "Séries liées",
+  "dashboard.seriesUnlinked": "Séries non liées",
+  "dashboard.byProvider": "Par fournisseur",
+  "dashboard.bookMetadata": "Métadonnées livres",
+  "dashboard.withSummary": "Avec résumé",
+  "dashboard.withIsbn": "Avec ISBN",
 
   // Books page
   "books.title": "Livres",