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/cmd/backend/main.go

@@ -189,6 +189,7 @@ func run() error {
 			ChapterImageStore: store,
 			Producer:          producer,
 			TaskReader:        store,
+			ImportFileStore:   store,
 			SearchIndex:       searchIndex,
 			Kokoro:            kokoroClient,
 			PocketTTS:         pocketTTSClient,