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 {