Fail GitHub scans on asset errors and drop sync state with its remote

- fail the scan when a release asset fails to derive or insert, so the ETag is kept and the retry re-derives it
- cascade sync-state rows on remote delete (migration 0003)

internal/provider/deb/github.go

@@ -333,6 +333,9 @@ func (p *GitHubProvider) scanWithState(ctx context.Context, remote models.Remote
 		return newEtag, false, err
 	}
 
+	// A failed asset fails the scan so the ETag is not advanced and the retry
+	// re-derives it; assets that did derive are kept and served meanwhile.
+	var failed []error
 	seen := map[string]bool{}
 	for _, rel := range releases {
 		if rel.Draft {
@@ -361,10 +364,12 @@ func (p *GitHubProvider) scanWithState(ctx context.Context, remote models.Remote
 			meta, err := p.deriveAsset(ctx, remote, asset, fp)
 			if err != nil {
 				slog.Warn("github_deb: derive asset failed", "remote", remote.Name, "asset", asset.Name, "error", err)
+				failed = append(failed, fmt.Errorf("%s: %w", asset.Name, err))
 				continue
 			}
 			if err := store.InsertDebMetadata(ctx, meta); err != nil {
 				slog.Error("github_deb: insert metadata failed", "remote", remote.Name, "asset", asset.Name, "error", err)
+				failed = append(failed, fmt.Errorf("%s: %w", asset.Name, err))
 				continue
 			}
 			slog.Info("github_deb: derived asset", "remote", remote.Name, "name", meta.Name, "version", meta.Version, "arch", meta.Architecture)
@@ -376,7 +381,7 @@ func (p *GitHubProvider) scanWithState(ctx context.Context, remote models.Remote
 			_ = store.DeleteDebMetadata(ctx, remote.Name, fp)
 		}
 	}
-	return newEtag, true, nil
+	return newEtag, true, errors.Join(failed...)
 }
 
 type ghRelease struct {