feat: add day/week/month period toggle for dashboard line charts

Add a period selector (day, week, month) to the reading activity and
books added charts. The API now accepts a ?period= query param and
returns gap-filled data using generate_series so all time slots appear
even with zero values. Labels are locale-aware (short month, weekday).

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

apps/backoffice/lib/i18n/fr.ts

@@ -68,7 +68,10 @@ const fr = {
   "dashboard.readingStatus": "Statut de lecture",
   "dashboard.byFormat": "Par format",
   "dashboard.byLibrary": "Par bibliothèque",
-  "dashboard.booksAdded": "Livres ajoutés (12 derniers mois)",
+  "dashboard.booksAdded": "Livres ajoutés",
+  "dashboard.periodDay": "Jour",
+  "dashboard.periodWeek": "Semaine",
+  "dashboard.periodMonth": "Mois",
   "dashboard.popularSeries": "Séries populaires",
   "dashboard.noSeries": "Aucune série pour le moment",
   "dashboard.unknown": "Inconnu",
@@ -82,7 +85,7 @@ const fr = {
   "dashboard.withIsbn": "Avec ISBN",
   "dashboard.currentlyReading": "En cours de lecture",
   "dashboard.recentlyRead": "Derniers livres lus",
-  "dashboard.readingActivity": "Activité de lecture (12 derniers mois)",
+  "dashboard.readingActivity": "Activité de lecture",
   "dashboard.pageProgress": "p. {{current}} / {{total}}",
   "dashboard.noCurrentlyReading": "Aucun livre en cours",
   "dashboard.noRecentlyRead": "Aucun livre lu récemment",