fix(backend): preserve omitted fields on file update

FileRepo.Update rewrites every editable column, so a PATCH that omitted a
field cleared it. Seed original_name and metadata — which have no always-
present input in the file editor — from the current file so a partial
update leaves them untouched instead of nulling them. The merge path builds
its own complete patch and calls the repo directly, so it is unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/service/file_service.go

@@ -286,7 +286,17 @@ func (s *FileService) Update(ctx context.Context, id uuid.UUID, p UpdateParams)
 		return nil, domain.ErrForbidden
 	}
 
-	patch := &domain.File{}
+	// The repo rewrites every editable column, so the patch must carry the final
+	// value for each. Seed the fields that have no always-present input in the
+	// editor (original_name, metadata) with their current values so a partial
+	// update that omits them leaves them untouched instead of clearing them. The
+	// remaining scalars are always supplied by the editor (notes uses an explicit
+	// null to clear). The merge path builds its own complete patch and calls the
+	// repo directly, so it is unaffected by this.
+	patch := &domain.File{
+		OriginalName: f.OriginalName,
+		Metadata:     f.Metadata,
+	}
 	if p.OriginalName != nil {
 		patch.OriginalName = p.OriginalName
 	}