fix: v0.2 review sweep

Controller-ruled small fixes accumulated from T3/T4 reviews:

1. routes_media.go: multipart Content-Type check is now an exact
   mediaType != "multipart/form-data" comparison (HasPrefix admitted
   "multipart/form-datax"); a malformed-tail multipart body (fails to
   parse at all) now reports "malformed multipart body", distinct from
   "model is required" (a body that parses fine but carries no model
   part) and "duplicate model field".
2. provider_openai.go: audioTranscription's doc comment no longer claims
   the body is always "replayed unchanged"/the boundary always
   "verbatim" — it now reflects the rewrite path routes_media.go
   already takes for an aliased model id.
3. rewriteMultipartModel: buf.Grow(len(body)+512) up front, avoiding
   repeated reallocate-and-copy growth while rebuilding an aliased
   multipart body.
4. limits.go currentUsage: guards len(vals) == len(keys) before indexing
   storeGetMulti's result, so a non-conforming counterStore's short slice
   reports storeDown instead of panicking.
5. admin.go dashboard JS: the 5s poll no longer wipes an existing
   auth-error message when no key is stored (only a successful load or a
   new submission clears it now); clearStoredKey on a 401/403 now only
   fires when the failing request's own key still matches what's
   currently stored, guarding against a late response for a key the user
   has since replaced.
6. Spec §4's duplicated /admin/api/overview bullet and the
   nosniff/no-store sentence's placement were already fixed upstream
   (docs/superpowers/specs/2026-08-20-llmgateway-v02-design.md, commit
   c02010d) — verified identical, nothing further to commit here.
7. Added TestAdmin_JSONSecurityHeaders (nosniff/no-store/CSP on both
   admin JSON APIs) and TestHandleAudioTranscriptions_MalformedTail_400
   (covers fix 1's new malformed-body branch).

go test ./... -count=1 -race, gofmt, vet, make yaegi-check, golangci-lint
(incl. gosec) all pass.

admin.go

@@ -590,17 +590,35 @@ const adminPageHTML = `<!doctype html>
     s.className = isErr ? "err" : "";
   }
 
+  // showAuthGate switches to the key-entry view. message is OPTIONAL
+  // (review sweep, 2026-08-20): passed with a string, it replaces the
+  // auth-error text; passed as undefined (refresh's own no-stored-key
+  // branch), it leaves whatever error text is already there untouched —
+  // otherwise a 5s poll running while no key is stored (the state right
+  // after a 401 already showed "invalid key, or not an admin") would call
+  // showAuthGate() on every tick and silently wipe that message back to
+  // empty before the user ever reads it.
   function showAuthGate(message) {
     document.getElementById("dashboard").classList.add("hidden");
     document.getElementById("auth-gate").classList.remove("hidden");
-    document.getElementById("auth-error").textContent = message || "";
+    if (message !== undefined) {
+      document.getElementById("auth-error").textContent = message;
+    }
   }
 
   function showDashboard() {
     document.getElementById("auth-gate").classList.add("hidden");
     document.getElementById("dashboard").classList.remove("hidden");
+    document.getElementById("auth-error").textContent = "";
   }
 
+  // fetchJSON captures the key it was actually sent with (key) onto a
+  // thrown auth error, rather than reading getStoredKey() again at the
+  // catch site: refresh's own Promise.all can still have this request's
+  // 401 in flight after the user has already submitted a newer key
+  // (setStoredKey then a fresh refresh() call) — capturing at throw time
+  // lets the catch handler tell that late, stale-key failure apart from a
+  // genuine rejection of the key currently stored.
   function fetchJSON(url) {
     var key = getStoredKey();
     var headers = key ? { "x-api-key": key } : {};
@@ -608,6 +626,7 @@ const adminPageHTML = `<!doctype html>
       if (resp.status === 401 || resp.status === 403) {
         var err = new Error("admin key rejected: HTTP " + resp.status);
         err.authFailed = true;
+        err.key = key;
         throw err;
       }
       if (!resp.ok) throw new Error(url + ": HTTP " + resp.status);
@@ -617,7 +636,7 @@ const adminPageHTML = `<!doctype html>
 
   function refresh() {
     if (!getStoredKey()) {
-      showAuthGate("");
+      showAuthGate();
       return;
     }
     Promise.all([fetchJSON(OVERVIEW_URL), fetchJSON(USAGE_URL)]).then(function (results) {
@@ -627,8 +646,15 @@ const adminPageHTML = `<!doctype html>
       setStatus("last updated " + new Date().toLocaleTimeString(), false);
     }).catch(function (err) {
       if (err.authFailed) {
-        clearStoredKey();
-        showAuthGate("invalid key, or not an admin");
+        // Guard against the late-401 race (review sweep, 2026-08-20): only
+        // clear the stored key and show the rejection when the FAILING
+        // request's own key still matches what is currently stored. A
+        // stale in-flight request for a key the user has since replaced
+        // must not clobber the newer key that may well be valid.
+        if (err.key === getStoredKey()) {
+          clearStoredKey();
+          showAuthGate("invalid key, or not an admin");
+        }
         return;
       }
       setStatus("refresh failed: " + err.message, true);
@@ -641,6 +667,7 @@ const adminPageHTML = `<!doctype html>
     var val = input.value;
     input.value = "";
     if (!val) return;
+    document.getElementById("auth-error").textContent = "";
     setStoredKey(val);
     refresh();
   });