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/storage/store.go

@@ -647,16 +647,17 @@ func (s *Store) CreateTranslationTask(ctx context.Context, slug string, chapter
 	return rec.ID, nil
 }
 
-func (s *Store) CreateImportTask(ctx context.Context, slug, title, fileType, objectKey string) (string, error) {
+func (s *Store) CreateImportTask(ctx context.Context, slug, title, fileType, objectKey, initiatorUserID string) (string, error) {
 	payload := map[string]any{
-		"slug":         slug,
-		"title":        title,
-		"file_name":    slug + "." + fileType,
-		"file_type":    fileType,
-		"status":       string(domain.TaskStatusPending),
-		"chapters_done": 0,
-		"chapters_total": 0,
-		"started":      time.Now().UTC().Format(time.RFC3339),
+		"slug":              slug,
+		"title":             title,
+		"file_name":         slug + "." + fileType,
+		"file_type":         fileType,
+		"status":            string(domain.TaskStatusPending),
+		"chapters_done":     0,
+		"chapters_total":    0,
+		"started":           time.Now().UTC().Format(time.RFC3339),
+		"initiator_user_id": initiatorUserID,
 	}
 	var rec struct {
 		ID string `json:"id"`
@@ -1134,6 +1135,7 @@ func parseImportTask(raw json.RawMessage) (domain.ImportTask, error) {
 		FileName        string `json:"file_name"`
 		FileType        string `json:"file_type"`
 		WorkerID        string `json:"worker_id"`
+		InitiatorUserID string `json:"initiator_user_id"`
 		Status          string `json:"status"`
 		ChaptersDone    int    `json:"chapters_done"`
 		ChaptersTotal   int    `json:"chapters_total"`
@@ -1153,6 +1155,7 @@ func parseImportTask(raw json.RawMessage) (domain.ImportTask, error) {
 		FileName:        rec.FileName,
 		FileType:        rec.FileType,
 		WorkerID:        rec.WorkerID,
+		InitiatorUserID: rec.InitiatorUserID,
 		Status:          domain.TaskStatus(rec.Status),
 		ChaptersDone:    rec.ChaptersDone,
 		ChaptersTotal:   rec.ChaptersTotal,