fix(deb): normalize apt flat-repo ./ dist prefix in github_deb serve

The metadata-only github_deb flat repo has the same defect as the local repo:
real apt requests <repo>/./Packages (and ./Release), which ServeRemote matched
literally and 404d. Reuse deb.go normalizeIndexPath to collapse the dot-segment
before matching the synthesized index.

internal/provider/deb/github.go

@@ -131,9 +131,13 @@ func (p *GitHubProvider) AuthHeaders(ctx context.Context, remote models.Remote)
 // (Packages/Packages.gz/Release), 404s the signed index variants (the repo is
 // consumed via [trusted=yes]), and 302-redirects .deb downloads to the backend
 // releases_remote. Returns false only for paths it does not own.
-func (p *GitHubProvider) ServeRemote(w http.ResponseWriter, r *http.Request, remote models.Remote, path, proxyBaseURL string, store provider.RemoteMetadataStore) bool {
+func (p *GitHubProvider) ServeRemote(w http.ResponseWriter, r *http.Request, remote models.Remote, reqPath, proxyBaseURL string, store provider.RemoteMetadataStore) bool {
 	p.onRequest(remote, store)
 
+	// apt appends the flat-repo dist "./" verbatim, so it asks for "./Packages"
+	// etc.; collapse the dot-segment before matching the synthesized index.
+	path := normalizeIndexPath(reqPath)
+
 	switch path {
 	case "Packages", "Packages.gz", "Release":
 		p.serveIndex(w, r, remote, path, store)