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/domain/domain.go

@@ -172,18 +172,19 @@ type TranslationResult struct {
 
 // ImportTask represents a PDF/EPUB import job stored in PocketBase.
 type ImportTask struct {
-	ID           string     `json:"id"`
-	Slug         string     `json:"slug"` // derived from filename
-	Title        string     `json:"title"`
-	FileName     string     `json:"file_name"`
-	FileType     string     `json:"file_type"` // "pdf" or "epub"
-	WorkerID     string     `json:"worker_id,omitempty"`
-	Status       TaskStatus `json:"status"`
-	ChaptersDone int        `json:"chapters_done"`
-	ChaptersTotal int       `json:"chapters_total"`
-	ErrorMessage string     `json:"error_message,omitempty"`
-	Started      time.Time  `json:"started"`
-	Finished     time.Time  `json:"finished,omitempty"`
+	ID               string     `json:"id"`
+	Slug             string     `json:"slug"` // derived from filename
+	Title            string     `json:"title"`
+	FileName         string     `json:"file_name"`
+	FileType         string     `json:"file_type"` // "pdf" or "epub"
+	WorkerID         string     `json:"worker_id,omitempty"`
+	InitiatorUserID  string     `json:"initiator_user_id,omitempty"` // PocketBase user ID who submitted the import
+	Status           TaskStatus `json:"status"`
+	ChaptersDone     int        `json:"chapters_done"`
+	ChaptersTotal    int        `json:"chapters_total"`
+	ErrorMessage     string     `json:"error_message,omitempty"`
+	Started          time.Time  `json:"started"`
+	Finished         time.Time  `json:"finished,omitempty"`
 }
 
 // ImportResult is the outcome reported by the runner after finishing an ImportTask.