v2.6.51: fix PDF import — raise UI body limit, wire real analyze

- docker-compose.yml: BODY_SIZE_LIMIT=52428800 (50MB) on UI service
  — adapter-node was rejecting PDFs >512KB with 'Content-length exceeds limit'
- storage/import.go: add AnalyzeFile() public function using real PDF/EPUB parsers
- handlers_import.go: analyzeImportFile() now calls storage.AnalyzeFile() instead
  of the file-size stub, so chapter count is accurate in the preview

backend/internal/backend/handlers_import.go

@@ -127,16 +127,17 @@ func (s *Server) handleAdminImport(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
-// analyzeImportFile does a quick scan of the file to count chapters.
-// This is a placeholder - real implementation would parse PDF/EPUB properly.
+// analyzeImportFile parses the file to count chapters and extract preview lines.
 func analyzeImportFile(data []byte, fileType string) *importPreview {
-	// TODO: Implement actual PDF/EPUB parsing to count chapters
-	// For now, estimate based on file size
-	preview := &importPreview{
-		Chapters:   estimateChapters(data, fileType),
-		FirstLines: []string{},
+	count, firstLines, err := storage.AnalyzeFile(data, fileType)
+	if err != nil || count == 0 {
+		// Fall back to rough size estimate so the UI still shows something
+		count = estimateChapters(data, fileType)
+	}
+	return &importPreview{
+		Chapters:   count,
+		FirstLines: firstLines,
 	}
-	return preview
 }
 
 func estimateChapters(data []byte, fileType string) int {

backend/internal/storage/import.go

@@ -54,7 +54,41 @@ func (i *importer) Import(ctx context.Context, objectKey, fileType string) ([]bo
 	return parseEPUB(data)
 }
 
-// ── PDF parsing ───────────────────────────────────────────────────────────────
+// AnalyzeFile parses the given PDF or EPUB data and returns the detected
+// chapter count and up to 3 preview lines (first non-empty line of each of
+// the first 3 chapters). It is used by the analyze-only endpoint so users
+// can preview chapter count before committing the import.
+func AnalyzeFile(data []byte, fileType string) (chapterCount int, firstLines []string, err error) {
+	var chapters []bookstore.Chapter
+	switch fileType {
+	case "pdf":
+		chapters, err = parsePDF(data)
+	case "epub":
+		chapters, err = parseEPUB(data)
+	default:
+		return 0, nil, fmt.Errorf("unsupported file type: %s", fileType)
+	}
+	if err != nil {
+		return 0, nil, err
+	}
+	chapterCount = len(chapters)
+	for i, ch := range chapters {
+		if i >= 3 {
+			break
+		}
+		line := strings.TrimSpace(ch.Content)
+		if nl := strings.Index(line, "\n"); nl > 0 {
+			line = line[:nl]
+		}
+		if len(line) > 120 {
+			line = line[:120] + "…"
+		}
+		firstLines = append(firstLines, line)
+	}
+	return chapterCount, firstLines, nil
+}
+
+
 
 func parsePDF(data []byte) ([]bookstore.Chapter, error) {
 	r, err := pdf.NewReader(bytes.NewReader(data), int64(len(data)))