feat: unified chat and embeddings endpoints with limits and accounting

Implements Task 12: POST /v1/chat/completions and POST /v1/embeddings
through a shared pipeline (routes_unified.go: handleChat, handleEmbeddings,
runUnified) wired into ServeHTTP's dispatch in llmgateway.go.

Pipeline: decode (10MiB cap) -> resolve model -> enforce user/group
limits -> call adapter -> account usage, translating every failure mode
into its HTTP envelope per the task brief's carried rulings:

- ALIAS ECHO (ruling a): the client-requested model id is threaded to the
  adapter via the internal gatewayAliasKey convention and echoed back in
  anthropic/gemini's translated response model field; every adapter
  strips the key before building its upstream wire body so it never
  leaks to a real provider.
- Limit violations (ruling b): storeDown -> 503 server_error; otherwise
  429 rate_limit_error with Retry-After when the violation names a
  window.
- Stream-safe error handling (ruling c): statusTrackingWriter.wroteHeader
  gates whether an adapter error still gets an envelope; providerHTTPError
  wraps into {error:{message,type,code,upstream}}; translateError maps to
  400/501; context.Canceled logs quietly; anything else is a 502.
- Usage/accounting (ruling d): zero non-streaming usage gets a
  ceil(len(body)/4) prompt-token estimate; zero streaming usage logs once
  and accounts the request only. unifiedCostMicros extends costMicros'
  single-id lookup to canonical-then-bare without changing its signature.
- Scopes (ruling e): user scope only when u.limits is set, group scope
  only when grp.limits is set, user first.

routes_unified_test.go drives every case end-to-end through
Gateway.ServeHTTP against httptest fake upstreams (openai- and
anthropic-shaped): happy path with counters asserted via the limiter's
own store, incremental stream flushing, 401/403/404/429/503, embeddings,
alias echo through a provider-prefixed anthropic request, wrapped
upstream error envelopes, and a mid-stream drop that must not append a
second envelope to an already-committed response.

Gates: gofmt, go vet, fieldalignment, golangci-lint (incl. gosec) all
clean; go test ./... -race -count=1 passes.

llmgateway.go

@@ -254,6 +254,20 @@ func (g *Gateway) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if r.Method == http.MethodPost && (r.URL.Path == "/v1/chat/completions" || r.URL.Path == "/v1/embeddings") {
+		u, grp, ok := g.auth.identify(r)
+		if !ok {
+			writeOAIError(sw, http.StatusUnauthorized, "authentication_error", "invalid or missing API key")
+			return
+		}
+		if r.URL.Path == "/v1/chat/completions" {
+			g.handleChat(sw, r, u, grp)
+		} else {
+			g.handleEmbeddings(sw, r, u, grp)
+		}
+		return
+	}
+
 	if g.cfg.PassthroughUnknown {
 		g.next.ServeHTTP(sw, r)
 		return