fix(import): wire ImportFileStore to bypass Asynq type assertion; add pb-init collections

- Add ImportFileStore interface to bookstore package
- Add ImportFileStore field to backend.Dependencies
- Wire ImportFileStore: store in cmd/backend/main.go
- handlers_import.go: use s.deps.ImportFileStore.PutImportFile instead of
  broken s.deps.Producer.(*storage.Store) type assertion (fails when Asynq active)
- pb-init-v3.sh: add import_tasks and notifications collection definitions

backend/internal/backend/handlers_import.go

@@ -98,12 +98,11 @@ func (s *Server) handleAdminImport(w http.ResponseWriter, r *http.Request) {
 
 		// Upload to MinIO for actual import
 		objectKey = fmt.Sprintf("imports/%d_%s", time.Now().Unix(), header.Filename)
-		store, ok := s.deps.Producer.(*storage.Store)
-		if !ok {
+		if s.deps.ImportFileStore == nil {
 			jsonError(w, http.StatusInternalServerError, "storage not available")
 			return
 		}
-		if err := store.PutImportFile(r.Context(), objectKey, data); err != nil {
+		if err := s.deps.ImportFileStore.PutImportFile(r.Context(), objectKey, data); err != nil {
 			jsonError(w, http.StatusInternalServerError, "upload file: "+err.Error())
 			return
 		}

backend/internal/backend/server.go

@@ -85,6 +85,9 @@ type Dependencies struct {
 	// BookWriter writes book metadata and chapter refs to PocketBase.
 	// Used by admin text-gen apply endpoints.
 	BookWriter bookstore.BookWriter
+	// ImportFileStore uploads raw PDF/EPUB files to MinIO for the runner to process.
+	// Always wired to the concrete *storage.Store (not the Asynq wrapper).
+	ImportFileStore bookstore.ImportFileStore
 	// AIJobStore tracks long-running AI generation jobs in PocketBase.
 	// If nil, job persistence is disabled (jobs still run but are not recorded).
 	AIJobStore bookstore.AIJobStore

backend/internal/bookstore/bookstore.go

@@ -215,3 +215,10 @@ type BookImporter interface {
 	// Returns the extracted chapters or an error.
 	Import(ctx context.Context, objectKey, fileType string) ([]Chapter, error)
 }
+
+// ImportFileStore uploads raw import files to object storage.
+// Kept separate from BookImporter so the HTTP handler can upload the file
+// without a concrete type assertion, regardless of which Producer is wired.
+type ImportFileStore interface {
+	PutImportFile(ctx context.Context, objectKey string, data []byte) error
+}