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/asynqqueue/producer.go

@@ -96,11 +96,12 @@ func (p *Producer) CreateImportTask(ctx context.Context, task domain.ImportTask)
 	}
 
 	payload := ImportPayload{
-		PBTaskID:  id,
-		Slug:      task.Slug,
-		Title:     task.Title,
-		FileType:  task.FileType,
-		ObjectKey: task.ObjectKey,
+		PBTaskID:    id,
+		Slug:        task.Slug,
+		Title:       task.Title,
+		FileType:    task.FileType,
+		ObjectKey:   task.ObjectKey,
+		ChaptersKey: task.ChaptersKey,
 	}
 	if err := p.enqueue(ctx, TypeImportBook, payload); err != nil {
 		// Non-fatal: PB record exists; runner will pick it up on next poll.

backend/internal/asynqqueue/tasks.go

@@ -48,9 +48,10 @@ type ScrapePayload struct {
 
 // ImportPayload is the Asynq job payload for PDF/EPUB import tasks.
 type ImportPayload struct {
-	PBTaskID  string `json:"pb_task_id"`
-	Slug      string `json:"slug"`
-	Title     string `json:"title"`
-	FileType  string `json:"file_type"` // "pdf" or "epub"
-	ObjectKey string `json:"object_key"` // MinIO path to uploaded file
+	PBTaskID    string `json:"pb_task_id"`
+	Slug        string `json:"slug"`
+	Title       string `json:"title"`
+	FileType    string `json:"file_type"`    // "pdf" or "epub"
+	ObjectKey   string `json:"object_key"`   // MinIO path to uploaded file
+	ChaptersKey string `json:"chapters_key"` // MinIO path to pre-parsed chapters JSON
 }