feat(cag): CAG integration — memory adapter, warmup semaphore, cache-aware routing (#47)

## Summary

Three issues in one PR. Implements Cache-Augmented Generation (CAG) support in the api-gw.

---

### #44 — Memory ServiceAdapter: standing-context resource

- Code-registers `memory` adapter with `standing-context` resource (`POST /memory/standing-context`)
- Fixes dispatcher bug: method-level `Auth.Capability` override now respected (was always using verb-derived capability)
- Auth: `memory:read` required via method-level override; `memory:write` for ingest
- `MEMORY_UPSTREAM_URL` env configures upstream

### #45 — Continuous batching: warmup deprioritisation + per-backend semaphore

- In-memory bounded queue per backend: `VLLM_MAX_QUEUE_DEPTH` slots (default 20)
- Concurrency limiter: `VLLM_MAX_NUM_SEQS` slots (default 4)
- `X-Poimen-Warmup: true` → sleep `WARMUP_DELAY_MS` (default 500ms) before queue entry, strip header, add `X-Priority: low`
- Queue full → 503 immediately, no goroutine pile-up
- Metrics: `warmup_requests_delayed_total`, `queue_full_requests_total` at `/metrics`

### #46 — Cache-aware routing: rendezvous hash + prefix cache hit logging

- `addresses: [...]` YAML field for multi-backend models (backward-compat with `address`)
- `X-Poimen-Cache-Key` header → rendezvous-hash backend selection (same key = same GPU)
- No cache key → round-robin
- Failed backend excluded 30s; next-highest-score fallback
- `vllm-num-cached-tokens` response header captured → `vllm_prefix_cache_hit_tokens_total` counter + structured log

---

## Verification

```
go build ./...    ✅
go vet ./...      ✅
go test -race ./... ✅ (all packages, 0 races)
```

## Reviewer verdicts

- reviewer-sre: SIGN-OFF
- reviewer-quality: SIGN-OFF

## Non-blocking findings (🟡🔵 only)

- `getOrCreateTransport` not goroutine-safe by design; pre-population invariant documented
- `time.Sleep(warmupDelay)` holds goroutine; context-cancel alternative left as TODO
- OTel trace_id not propagated into CAG log events (existing gap)

Closes #44 Closes #45 Closes #46

---------

Co-authored-by: Admin Bot <[email protected]>
Reviewed-on: #47

internal/server/router.go

@@ -16,6 +16,7 @@ type Router struct {
 	temporalHandler   http.Handler
 	upstreamHandler   http.Handler
 	forgejoWebhook    *webhook.ForgejoHandler
+	metricsHandler    http.Handler
 }
 
 // NewRouter creates a new router with health endpoints.
@@ -33,6 +34,11 @@ func NewRouter(healthChecker *HealthChecker, dispatcher *serviceadapter.Dispatch
 	}
 }
 
+// SetMetricsHandler registers the Prometheus /metrics endpoint handler.
+func (r *Router) SetMetricsHandler(h http.Handler) {
+	r.metricsHandler = h
+}
+
 // ServeHTTP implements http.Handler.
 // Priority order:
 // 1. /healthz and /readyz to health handlers
@@ -48,6 +54,11 @@ func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	case "/readyz":
 		ReadinessHandler(r.healthChecker)(w, req)
 		return
+	case "/metrics":
+		if r.metricsHandler != nil {
+			r.metricsHandler.ServeHTTP(w, req)
+			return
+		}
 	}
 
 	// X-Service (ServiceAdapter) routing - checked before path-based routing