fix(import): persist object_key + metadata; add nav + logout session cleanup

- Import task: persist object_key, author, cover_url, genres, summary,
  book_status in PocketBase so the runner can fetch the file and write
  book metadata on completion
- Runner poll mode: pass task.ObjectKey instead of empty string
- Runner: write BookMeta + UpsertBook in Meilisearch after chapter ingest
  so imported books appear in catalogue and search
- Import UI: add author, cover URL, genres, summary, status fields; add
  AI tasks panel (chapter names, description, image gen, tagline) after
  import completes; add AI tasks button on each done task in the list
- Admin nav: add Notifications entry to sidebar (all 5 locales)
- Logout: delete user_sessions row on sign-out so sessions don't
  accumulate as phantoms after each login/logout cycle

ui/messages/fr.json

@@ -378,6 +378,7 @@
   "admin_nav_text_gen": "Text Gen",
   "admin_nav_catalogue_tools": "Catalogue Tools",
   "admin_nav_ai_jobs": "Tâches IA",
+  "admin_nav_notifications": "Notifications",
   "admin_nav_errors": "Erreurs",
   "admin_nav_analytics": "Analytique",
   "admin_nav_logs": "Journaux",