fix(worker): key rate limiter on peer IP, not forged headers

The rate limiter trusted X-Real-IP, so a client could dodge its limit by
sending a new header value on each request. It must key on the connection
peer address. The key also strips the port, so one client shares one bucket.

internal/worker/ratelimit_test.go

@@ -0,0 +1,35 @@
+package worker
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestPerClientRateLimitMiddleware_KeysOnPeerIP(t *testing.T) {
+	h := PerClientRateLimitMiddleware(NewPerClientRateLimiter(0.001, 1))(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) { w.WriteHeader(http.StatusOK) }))
+
+	do := func(remote, realIP string) int {
+		req := httptest.NewRequest(http.MethodGet, "/", nil)
+		req.RemoteAddr = remote
+		if realIP != "" {
+			req.Header.Set("X-Real-IP", realIP)
+		}
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, req)
+		return rec.Code
+	}
+
+	if got := do("10.0.0.1:1111", "1.1.1.1"); got != http.StatusOK {
+		t.Fatalf("first request: got %d", got)
+	}
+	// Same peer, new port and forged header: must share the exhausted bucket.
+	if got := do("10.0.0.1:2222", "2.2.2.2"); got != http.StatusTooManyRequests {
+		t.Fatalf("spoofed header/new port bypassed limit: got %d", got)
+	}
+	// Different peer is unaffected.
+	if got := do("10.0.0.2:1111", ""); got != http.StatusOK {
+		t.Fatalf("other peer: got %d", got)
+	}
+}