Pin the rate-limit key for an empty RemoteAddr (closes #168)

A request whose RemoteAddr is empty has no peer identity, so the rate limiters' key falls back to the raw empty string and every such request shares one bucket: it fails closed rather than giving each its own. net/http always fills RemoteAddr for a TCP listener, so normal serving never reaches this. The behaviour is unchanged and now deliberate: a test pins the shared key, and a one-sentence comment at the fallback tells the empty case apart from a Unix-socket listener, where every peer legitimately carries the same address.

Model: opus-5-5

internal/middleware/ratelimit.go

@@ -219,7 +219,11 @@ func (m *Middleware) clientKey(r *http.Request) string {
 		// path cannot silently collapse unrelated clients
 		// together. On a Unix-socket listener every peer
 		// carries the same RemoteAddr and so shares one bucket,
-		// which is the fail-closed direction.
+		// which is the fail-closed direction. An empty RemoteAddr
+		// is a different case, which net/http never produces for
+		// a TCP listener and only a hand-built request carries,
+		// but it fails closed the same way: every such request
+		// shares the one bucket keyed on the empty string.
 		return r.RemoteAddr
 	}