Fix github_alpine .apk redirect to resolve stored FilePath

apk reconstructs the download URL itself as <arch>/<name>-<version>.apk
because APKINDEX carries no filename field (unlike rpm's <location> or
deb's Filename:). ServeRemote forwarded that synthesized path verbatim
into the releases_remote redirect, pointing at a nonexistent,
allowlist-denied github.com path (404/403).

Look up the cached metadata row by arch plus the full reconstructed
filename (no hyphen-split, so -rN suffixes are preserved) and redirect
to the stored github-relative FilePath. Unknown packages now 404 instead
of redirecting to a bad path.

internal/provider/alpine/github.go

@@ -146,14 +146,49 @@ func (p *GitHubProvider) ServeRemote(w http.ResponseWriter, r *http.Request, rem
 			http.Error(w, "github_alpine remote has no releases_remote configured for downloads", http.StatusInternalServerError)
 			return true
 		}
-		loc := strings.TrimRight(proxyBaseURL, "/") + "/api/v1/remote/" + remote.ReleasesRemote + "/" + strings.TrimLeft(path, "/")
-		http.Redirect(w, r, loc, http.StatusFound)
+		p.serveApkRedirect(w, r, remote, path, proxyBaseURL, store)
 		return true
 	}
 
 	return false
 }
 
+// serveApkRedirect resolves an apk-reconstructed download path — apk builds
+// "<arch>/<name>-<version>.apk" itself because APKINDEX carries no filename — to
+// the real github-relative asset path stored on the metadata row, then redirects
+// to the backend releases_remote. Passing the inbound path through verbatim would
+// point at a nonexistent, allowlist-denied github.com path.
+func (p *GitHubProvider) serveApkRedirect(w http.ResponseWriter, r *http.Request, remote models.Remote, path, proxyBaseURL string, store provider.RemoteMetadataStore) {
+	arch := strings.TrimSuffix(path[:strings.LastIndex(path, "/")+1], "/")
+	basename := path[strings.LastIndex(path, "/")+1:]
+	if arch == "" || strings.Contains(arch, "/") {
+		http.Error(w, "apk download must be requested per-arch: <arch>/<name>-<version>.apk", http.StatusNotFound)
+		return
+	}
+
+	reader, ok := store.(provider.AlpineMetadataReader)
+	if !ok {
+		http.Error(w, "alpine metadata not available", http.StatusInternalServerError)
+		return
+	}
+	sctx, cancel := context.WithTimeout(context.WithoutCancel(r.Context()), p.serveTimeout)
+	defer cancel()
+	rows, err := reader.ListAlpineMetadataEntries(sctx, remote.Name)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	for _, row := range rows {
+		if row.Arch == arch && row.Name+"-"+row.Version+".apk" == basename {
+			loc := strings.TrimRight(proxyBaseURL, "/") + "/api/v1/remote/" + remote.ReleasesRemote + "/" + strings.TrimLeft(row.FilePath, "/")
+			http.Redirect(w, r, loc, http.StatusFound)
+			return
+		}
+	}
+	http.Error(w, "package not found", http.StatusNotFound)
+}
+
 func (p *GitHubProvider) serveIndex(w http.ResponseWriter, r *http.Request, remote models.Remote, path string, store provider.RemoteMetadataStore) {
 	arch := strings.TrimSuffix(path, "APKINDEX.tar.gz")
 	arch = strings.Trim(arch, "/")