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/bookstore/bookstore.go

@@ -221,4 +221,8 @@ type BookImporter interface {
 // without a concrete type assertion, regardless of which Producer is wired.
 type ImportFileStore interface {
 	PutImportFile(ctx context.Context, objectKey string, data []byte) error
+	// PutImportChapters stores the pre-parsed chapters JSON under the given key.
+	PutImportChapters(ctx context.Context, key string, data []byte) error
+	// GetImportChapters retrieves the pre-parsed chapters JSON.
+	GetImportChapters(ctx context.Context, key string) ([]byte, error)
 }