fix(library): sync allowed extensions from Go source of truth to DB on startup

AllowedExtensions in Go was the intended single source of truth for library
type file extensions, but it was never synced to the database. This caused
missing extensions like .pdf for manga to be absent from library_types.

- Add SyncAllowedExtensions() to sync Go AllowedExtensions map to DB
- Call SyncAllowedExtensions() from cmd/server/main.go on startup
- Ensure .pdf is included in manga extensions

internal/services/library_service.go

@@ -4,6 +4,7 @@ import (
 	"bookhoard/internal/database"
 	"context"
 	"fmt"
+	"log"
 	"os"
 	"path/filepath"
 	"strings"
@@ -286,6 +287,19 @@ func (s *LibraryService) BrowseDirectories(ctx context.Context, path string) ([]
 	return dirs, cleanPath, parentPath, nil
 }
 
+// SyncAllowedExtensions syncs the Go AllowedExtensions map into the database.
+// This ensures library_types.allowed_extensions stays in sync with the Go source of truth.
+func (s *LibraryService) SyncAllowedExtensions(ctx context.Context) {
+	for typeName, exts := range AllowedExtensions {
+		if err := s.db.SyncLibraryTypeExtensions(ctx, database.SyncLibraryTypeExtensionsParams{
+			Name:              typeName,
+			AllowedExtensions: exts,
+		}); err != nil {
+			log.Printf("Warning: failed to sync allowed extensions for library type %s: %v", typeName, err)
+		}
+	}
+}
+
 func (s *LibraryService) ResolveMediaPath(ctx context.Context, libraryID pgtype.UUID, relativePath string) (string, error) {
 	// Get library folders for this library
 	folders, err := s.db.GetLibraryFolders(ctx, libraryID)