feat(import): move PDF parsing to backend; fix heartbeat/reap for import_tasks

- parsePDF function restored in import.go (body was orphaned outside function)
- ParseImportFile() called at upload time with 3-min timeout; chapters stored as JSON in MinIO
- runner.go: prefer ChaptersKey path (read pre-parsed JSON) over BookImport.Import()
- ImportChapterStore interface added; store wired in runner/main.go
- HeartbeatTask and ReapStaleTasks now include import_tasks collection
- parseImportTask now returns ChaptersKey in domain.ImportTask
- asynq_runner.go handleImportTask passes ChaptersKey
- pb-init-v3.sh: chapters_key field added to import_tasks schema

backend/internal/runner/asynq_runner.go

@@ -199,10 +199,11 @@ func (r *Runner) handleImportTask(ctx context.Context, t *asynq.Task) error {
 		return fmt.Errorf("unmarshal import payload: %w", err)
 	}
 	task := domain.ImportTask{
-		ID:        p.PBTaskID,
-		Slug:      p.Slug,
-		Title:     p.Title,
-		FileType:  p.FileType,
+		ID:          p.PBTaskID,
+		Slug:        p.Slug,
+		Title:       p.Title,
+		FileType:    p.FileType,
+		ChaptersKey: p.ChaptersKey,
 	}
 	r.tasksRunning.Add(1)
 	defer r.tasksRunning.Add(-1)