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/api.ts

@@ -494,6 +494,20 @@ export type MonthlyAdditions = {
   books_added: number;
 };
 
+export type ProviderCount = {
+  provider: string;
+  count: number;
+};
+
+export type MetadataStats = {
+  total_series: number;
+  series_linked: number;
+  series_unlinked: number;
+  books_with_summary: number;
+  books_with_isbn: number;
+  by_provider: ProviderCount[];
+};
+
 export type StatsResponse = {
   overview: StatsOverview;
   reading_status: ReadingStatusStats;
@@ -502,6 +516,7 @@ export type StatsResponse = {
   by_library: LibraryStatsItem[];
   top_series: TopSeriesItem[];
   additions_over_time: MonthlyAdditions[];
+  metadata: MetadataStats;
 };
 
 export async function fetchStats() {

apps/backoffice/lib/i18n/en.ts

@@ -75,6 +75,13 @@ const en: Record<TranslationKey, string> = {
   "dashboard.noSeries": "No series yet",
   "dashboard.unknown": "Unknown",
   "dashboard.readCount": "{{read}}/{{total}} read",
+  "dashboard.metadataCoverage": "Metadata coverage",
+  "dashboard.seriesLinked": "Linked series",
+  "dashboard.seriesUnlinked": "Unlinked series",
+  "dashboard.byProvider": "By provider",
+  "dashboard.bookMetadata": "Book metadata",
+  "dashboard.withSummary": "With summary",
+  "dashboard.withIsbn": "With ISBN",
 
   // Books page
   "books.title": "Books",

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",