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/app/page.tsx

@@ -137,7 +137,7 @@ export default async function DashboardPage() {
     );
   }
 
-  const { overview, reading_status, by_format, by_language, by_library, top_series, additions_over_time } = stats;
+  const { overview, reading_status, by_format, by_language, by_library, top_series, additions_over_time, metadata } = stats;
 
   const readingColors = ["hsl(220 13% 70%)", "hsl(45 93% 47%)", "hsl(142 60% 45%)"];
   const formatColors = [
@@ -231,6 +231,69 @@ export default async function DashboardPage() {
         </Card>
       </div>
 
+      {/* Metadata row */}
+      <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-6">
+        {/* Series metadata coverage donut */}
+        <Card hover={false}>
+          <CardHeader>
+            <CardTitle className="text-base">{t("dashboard.metadataCoverage")}</CardTitle>
+          </CardHeader>
+          <CardContent>
+            <DonutChart
+              locale={locale}
+              noDataLabel={noDataLabel}
+              data={[
+                { label: t("dashboard.seriesLinked"), value: metadata.series_linked, color: "hsl(142 60% 45%)" },
+                { label: t("dashboard.seriesUnlinked"), value: metadata.series_unlinked, color: "hsl(220 13% 70%)" },
+              ]}
+            />
+          </CardContent>
+        </Card>
+
+        {/* By provider donut */}
+        <Card hover={false}>
+          <CardHeader>
+            <CardTitle className="text-base">{t("dashboard.byProvider")}</CardTitle>
+          </CardHeader>
+          <CardContent>
+            <DonutChart
+              locale={locale}
+              noDataLabel={noDataLabel}
+              data={metadata.by_provider.map((p, i) => ({
+                label: p.provider.replace(/_/g, " ").replace(/\b\w/g, (c) => c.toUpperCase()),
+                value: p.count,
+                color: formatColors[i % formatColors.length],
+              }))}
+            />
+          </CardContent>
+        </Card>
+
+        {/* Book metadata quality */}
+        <Card hover={false}>
+          <CardHeader>
+            <CardTitle className="text-base">{t("dashboard.bookMetadata")}</CardTitle>
+          </CardHeader>
+          <CardContent>
+            <div className="space-y-4">
+              <HorizontalBar
+                label={t("dashboard.withSummary")}
+                value={metadata.books_with_summary}
+                max={overview.total_books}
+                subLabel={overview.total_books > 0 ? `${Math.round((metadata.books_with_summary / overview.total_books) * 100)}%` : "0%"}
+                color="hsl(198 78% 37%)"
+              />
+              <HorizontalBar
+                label={t("dashboard.withIsbn")}
+                value={metadata.books_with_isbn}
+                max={overview.total_books}
+                subLabel={overview.total_books > 0 ? `${Math.round((metadata.books_with_isbn / overview.total_books) * 100)}%` : "0%"}
+                color="hsl(280 60% 50%)"
+              />
+            </div>
+          </CardContent>
+        </Card>
+      </div>
+
       {/* Second row */}
       <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
         {/* Monthly additions bar chart */}

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",