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/usage.go

@@ -0,0 +1,118 @@
+package aiproxy
+
+import (
+	"bytes"
+	"io"
+	"sync"
+
+	"github.com/tidwall/gjson"
+)
+
+const maxCapture = 1 << 20
+
+// watchBody passes the response through while it looks for OpenAI usage, and fires
+// AfterProxy once the body ends or is closed.
+type watchBody struct {
+	rc      io.ReadCloser
+	capture bool // read usage
+	sse     bool
+	status  int
+	c       *call
+
+	buf      bytes.Buffer // JSON body (capped)
+	overflow bool
+	line     []byte // partial SSE line
+	usage    Usage
+	once     sync.Once
+}
+
+func newWatchBody(rc io.ReadCloser, openai, sse bool, status int, c *call) io.ReadCloser {
+	return &watchBody{rc: rc, capture: openai, sse: sse, status: status, c: c}
+}
+
+func (w *watchBody) Read(p []byte) (int, error) {
+	n, err := w.rc.Read(p)
+	if w.capture && n > 0 {
+		w.feed(p[:n])
+	}
+	if err != nil {
+		w.end(err)
+	}
+	return n, err
+}
+
+func (w *watchBody) Close() error {
+	err := w.rc.Close()
+	w.end(nil)
+	return err
+}
+
+func (w *watchBody) end(err error) {
+	w.once.Do(func() {
+		if w.capture && !w.sse && !w.overflow {
+			if u, ok := parseUsage(w.buf.Bytes()); ok {
+				w.usage = u
+			}
+		}
+		if err == io.EOF {
+			err = nil
+		}
+		w.c.finish(w.status, w.usage, err)
+	})
+}
+
+func (w *watchBody) feed(b []byte) {
+	if !w.sse {
+		if w.overflow || w.buf.Len()+len(b) > maxCapture {
+			w.overflow = true
+			return
+		}
+		w.buf.Write(b)
+		return
+	}
+	w.line = append(w.line, b...)
+	for {
+		i := bytes.IndexByte(w.line, '\n')
+		if i < 0 {
+			break
+		}
+		line := bytes.TrimSpace(w.line[:i])
+		w.line = w.line[i+1:]
+		if rest, ok := bytes.CutPrefix(line, []byte("data:")); ok && bytes.Contains(rest, []byte("usage")) {
+			if u, ok := parseUsage(bytes.TrimSpace(rest)); ok {
+				w.usage = u
+			}
+		}
+	}
+	if len(w.line) > maxCapture {
+		w.line = nil
+	}
+}
+
+// parseUsage reads chat/embeddings ("usage") and responses ("response.usage") shapes.
+func parseUsage(data []byte) (Usage, bool) {
+	u := gjson.GetBytes(data, "usage")
+	if !u.IsObject() {
+		u = gjson.GetBytes(data, "response.usage")
+	}
+	if !u.IsObject() {
+		return Usage{}, false
+	}
+	first := func(keys ...string) int64 {
+		for _, k := range keys {
+			if v := u.Get(k); v.Exists() {
+				return v.Int()
+			}
+		}
+		return 0
+	}
+	out := Usage{
+		PromptTokens:     first("prompt_tokens", "input_tokens"),
+		CompletionTokens: first("completion_tokens", "output_tokens"),
+		TotalTokens:      first("total_tokens"),
+	}
+	if out.TotalTokens == 0 {
+		out.TotalTokens = out.PromptTokens + out.CompletionTokens
+	}
+	return out, true
+}