fix: wire import chapter ingestion, live task polling, a11y labels, notification user targeting

- NewBookImporter now takes *Store instead of raw *minio.Client (same package access)
- Runner Dependencies gains ChapterIngester interface; storeImportedChapters no-op removed
- store.IngestChapters is now actually called after PDF/EPUB extraction
- BookImport and ChapterIngester wired in runner main.go
- CreateImportTask interface gains initiatorUserID param; threaded through store/asynq/handler
- domain.ImportTask gains InitiatorUserID field; parseImportTask populates it
- Runner notifies initiator (falls back to 'admin' when empty)
- UI: import task list polls every 3s while any task is pending/running
- UI: label[for] + input[id] fix for a11y warnings in admin import page

backend/internal/taskqueue/taskqueue.go

@@ -35,7 +35,8 @@ type Producer interface {
 
 	// CreateImportTask inserts a new import task with status=pending and
 	// returns the assigned PocketBase record ID.
-	CreateImportTask(ctx context.Context, slug, title, fileType, objectKey string) (string, error)
+	// 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)
 
 	// CancelTask transitions a pending task to status=cancelled.
 	// Returns ErrNotFound if the task does not exist.