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

backend/internal/taskqueue/taskqueue.go

@@ -35,8 +35,8 @@ type Producer interface {
 
 	// CreateImportTask inserts a new import task with status=pending and
 	// returns the assigned PocketBase record ID.
-	// initiatorUserID is the PocketBase user ID who submitted the import (may be empty).
-	CreateImportTask(ctx context.Context, slug, title, fileType, objectKey, initiatorUserID string) (string, error)
+	// The task struct must have at minimum Slug, Title, FileType, and ObjectKey set.
+	CreateImportTask(ctx context.Context, task domain.ImportTask) (string, error)
 
 	// CancelTask transitions a pending task to status=cancelled.
 	// Returns ErrNotFound if the task does not exist.

backend/internal/taskqueue/taskqueue_test.go

@@ -26,7 +26,7 @@ func (s *stubStore) CreateAudioTask(_ context.Context, _ string, _ int, _ string
 func (s *stubStore) CreateTranslationTask(_ context.Context, _ string, _ int, _ string) (string, error) {
 	return "translation-1", nil
 }
-func (s *stubStore) CreateImportTask(_ context.Context, _, _, _, _, _ string) (string, error) {
+func (s *stubStore) CreateImportTask(_ context.Context, _ domain.ImportTask) (string, error) {
 	return "import-1", nil
 }
 func (s *stubStore) CancelTask(_ context.Context, _ string) error                    { return nil }