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/cmd/runner/main.go

@@ -192,21 +192,22 @@ func run() error {
 
 	deps := runner.Dependencies{
 		Consumer:         consumer,
-		BookWriter:       store,
-		BookReader:       store,
-		AudioStore:       store,
-		CoverStore:       store,
-		TranslationStore: store,
-		BookImport:       storage.NewBookImporter(store),
-		ChapterIngester:  store,
-		SearchIndex:      searchIndex,
-		Novel:            novel,
-		Kokoro:           kokoroClient,
-		PocketTTS:        pocketTTSClient,
-		CFAI:             cfaiClient,
-		LibreTranslate:   ltClient,
-		Notifier:         store,
-		Log:              log,
+		BookWriter:         store,
+		BookReader:         store,
+		AudioStore:         store,
+		CoverStore:         store,
+		TranslationStore:   store,
+		BookImport:         storage.NewBookImporter(store),
+		ImportChapterStore: store,
+		ChapterIngester:    store,
+		SearchIndex:        searchIndex,
+		Novel:              novel,
+		Kokoro:             kokoroClient,
+		PocketTTS:          pocketTTSClient,
+		CFAI:               cfaiClient,
+		LibreTranslate:     ltClient,
+		Notifier:           store,
+		Log:                log,
 	}
 	r := runner.New(rCfg, deps)