hub: resolve the two artifact dropdowns side by side, not one after the other

Follow-up to the fan-out, and the reason for it is worth recording: THE FIRST FIX DID LESS THAN THE
ARITHMETIC PREDICTED. Concurrency took the page from 26.2s to ~11-18s, not the ~2s expected, so the
gap was chased instead of declared closed.

What it found: the slowest single call on the page is not a per-version sha lookup at all, it is the
PACKAGE SEARCH (/api/v1/packages/admin?type=generic&q=...), measured in-cluster at 1.1-2.2s each
against ~0.24s for a file's metadata. Per-version fan-out cannot touch it — there is one search per
package and they ran in series.

The two dropdowns are independent, so they now resolve side by side, overlapping both searches and
both fan-outs. go test -race clean on the new concurrent paths.

Gitea's latency on this box is load-dependent and varies 2-4x between samples, so the CHANGELOG
quotes a range rather than a single pair of numbers.

hub/internal/web/server.go

@@ -906,6 +906,18 @@ func (s *Server) handleConfiguration(w http.ResponseWriter, r *http.Request) {
 	}
 
 	ctx := r.Context()
+
+	// The two dropdowns are independent, so they are resolved side by side rather than one after the
+	// other (v0.100.0). Each is internally concurrent already; doing them in series still cost the
+	// full latency of BOTH package searches, and that search is the slowest single call on this page
+	// — measured in-cluster at 1.1–2.2 s each against ~0.24 s for a file's metadata.
+	var agentChoices, goldenChoices []artifactChoice
+	var choicesWG sync.WaitGroup
+	choicesWG.Add(2)
+	go func() { defer choicesWG.Done(); agentChoices = s.artifactChoices(ctx, pkgAgent, fileAgent) }()
+	go func() { defer choicesWG.Done(); goldenChoices = s.artifactChoices(ctx, pkgGolden, fileGolden) }()
+	choicesWG.Wait()
+
 	data := map[string]interface{}{
 		"CSRFToken":     csrfToken,
 		"CSRFField":     s.csrfField(r),
@@ -914,8 +926,8 @@ func (s *Server) handleConfiguration(w http.ResponseWriter, r *http.Request) {
 		"GlobalFloor":   s.store.GetGlobalMinControllerVersion(),
 		"FloorRes":      s.store.ResolveGlobalFloor(),
 		"Artifacts":     s.store.GetArtifactManifest(),
-		"AgentChoices":  s.artifactChoices(ctx, pkgAgent, fileAgent),
-		"GoldenChoices": s.artifactChoices(ctx, pkgGolden, fileGolden),
+		"AgentChoices":  agentChoices,
+		"GoldenChoices": goldenChoices,
 		"Flash":         r.URL.Query().Get("flash"),
 	}
 	if err := s.templates.ExecuteTemplate(w, "configuration.html", data); err != nil {