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.

provider_openai.go

@@ -319,11 +319,14 @@ func (a *openaiAdapter) audioSpeech(ctx context.Context, w http.ResponseWriter,
 }
 
 // audioTranscription implements providerAdapter: a native forward to
-// {base}/v1/audio/transcriptions. body is the client's original raw
-// multipart request, replayed unchanged (routes_media.go never rewrites
-// it), and contentType is the client's original Content-Type header,
-// forwarded verbatim so the upstream sees the same multipart boundary the
-// client used. The response (JSON or plain text, depending on the
+// {base}/v1/audio/transcriptions. body is the multipart request
+// routes_media.go hands in — the client's original bytes when its "model"
+// field already named the resolved upstream model id, or a rewritten
+// body (a fresh boundary, every other part copied verbatim) when it did
+// not, per rewriteMultipartModel's controller ruling — and contentType
+// matches whichever body this call received, so the upstream always sees
+// a Content-Type whose boundary parameter is consistent with what
+// actually follows it. The response (JSON or plain text, depending on the
 // client's requested response_format) is copied through unchanged, the
 // same non-streaming path images.generations uses.
 func (a *openaiAdapter) audioTranscription(ctx context.Context, w http.ResponseWriter, body []byte, contentType string) (usage, error) {