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/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
+}