feat(aiproxy): add authenticated proxy for OpenAI-compatible APIs and MCP servers

Hides upstream URLs and keys behind the security layer. Per-upstream roles,
model/tool allowlists, per-user rate limits, Before/AfterProxy hooks, audit
sink, Prometheus metrics (metrics.AIProxyRecorder) and upstreams loaded from
the resolvespec_ai_proxies stored procedure.

pkg/aiproxy/ratelimit.go

@@ -0,0 +1,80 @@
+package aiproxy
+
+import (
+	"math"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/time/rate"
+)
+
+const (
+	limiterSweepEvery = 5 * time.Minute
+	limiterIdleAfter  = 10 * time.Minute
+)
+
+type limiterEntry struct {
+	lim  *rate.Limiter
+	last time.Time
+}
+
+// limiters keeps one token bucket per key and drops idle ones.
+type limiters struct {
+	mu        sync.Mutex
+	entries   map[string]*limiterEntry
+	lastSweep time.Time
+}
+
+func newLimiters() *limiters {
+	return &limiters{entries: make(map[string]*limiterEntry), lastSweep: time.Now()}
+}
+
+// allow takes a token. When denied it returns how long to wait.
+func (l *limiters) allow(key string, rl *RateLimit) (bool, time.Duration) {
+	now := time.Now()
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	if now.Sub(l.lastSweep) > limiterSweepEvery {
+		for k, e := range l.entries {
+			if now.Sub(e.last) > limiterIdleAfter {
+				delete(l.entries, k)
+			}
+		}
+		l.lastSweep = now
+	}
+
+	e, ok := l.entries[key]
+	if !ok {
+		burst := rl.Burst
+		if burst < 1 {
+			burst = int(math.Ceil(rl.PerSecond))
+			if burst < 1 {
+				burst = 1
+			}
+		}
+		e = &limiterEntry{lim: rate.NewLimiter(rate.Limit(rl.PerSecond), burst)}
+		l.entries[key] = e
+	}
+	e.last = now
+
+	res := e.lim.ReserveN(now, 1)
+	if d := res.DelayFrom(now); d > 0 {
+		res.CancelAt(now)
+		return false, d
+	}
+	return true, 0
+}
+
+// forget drops the buckets of an upstream (after it was replaced or removed).
+func (l *limiters) forget(upstream string) {
+	prefix := upstream + "|"
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	for k := range l.entries {
+		if strings.HasPrefix(k, prefix) {
+			delete(l.entries, k)
+		}
+	}
+}