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/storage/import.go

@@ -109,6 +109,40 @@ func decryptPDF(data []byte) ([]byte, error) {
 	return out.Bytes(), nil
 }
 
+// ParseImportFile parses a PDF or EPUB and returns chapters.
+// Unlike AnalyzeFile it respects ctx cancellation so callers can apply a timeout.
+// For PDFs it first attempts to strip encryption with an empty password.
+func ParseImportFile(ctx context.Context, data []byte, fileType string) ([]bookstore.Chapter, error) {
+	type result struct {
+		chapters []bookstore.Chapter
+		err      error
+	}
+	ch := make(chan result, 1)
+	go func() {
+		var chapters []bookstore.Chapter
+		var err error
+		switch fileType {
+		case "pdf":
+			chapters, err = parsePDF(data)
+		case "epub":
+			chapters, err = parseEPUB(data)
+		default:
+			err = fmt.Errorf("unsupported file type: %s", fileType)
+		}
+		ch <- result{chapters, err}
+	}()
+	select {
+	case <-ctx.Done():
+		return nil, fmt.Errorf("parse timed out: %w", ctx.Err())
+	case r := <-ch:
+		return r.chapters, r.err
+	}
+}
+
+// parsePDF extracts chapters from PDF bytes using dslipak/pdf.
+// It first attempts to decrypt the PDF with an empty password in case the file
+// uses owner-only encryption (copy/print restrictions), which is common for
+// publisher PDFs that open normally in readers but confuse raw parsers.
 func parsePDF(data []byte) ([]bookstore.Chapter, error) {
 	// If the PDF is encrypted, try to decrypt it with an empty password.
 	// Many publisher PDFs use owner-only encryption (copy/print restrictions)