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/domain/domain.go

@@ -178,6 +178,7 @@ type ImportTask struct {
 	FileName         string     `json:"file_name"`
 	FileType         string     `json:"file_type"` // "pdf" or "epub"
 	ObjectKey        string     `json:"object_key,omitempty"` // MinIO path to uploaded file
+	ChaptersKey      string     `json:"chapters_key,omitempty"` // MinIO path to pre-parsed chapters JSON
 	Author           string     `json:"author,omitempty"`
 	CoverURL         string     `json:"cover_url,omitempty"`
 	Genres           []string   `json:"genres,omitempty"`