feat(security): add trusted proxies and abuse event cleanup

- Add `WARPBOX_TRUSTED_PROXIES` configuration to restrict accepted forwarded client IP headers to specific proxy IPs/CIDRs, securing client IP resolution.
- Integrate `BanService` into the background cleanup job to automatically purge expired abuse and ban evidence events.
- Update documentation with reverse proxy security guidelines and a production systemd deployment guide.

backend/libs/services/proxy_test.go

@@ -0,0 +1,29 @@
+package services
+
+import "testing"
+
+func TestClientIPTrustsForwardedHeadersByDefault(t *testing.T) {
+	ip := ClientIP("127.0.0.1:6070", "203.0.113.10, 10.0.0.2", "198.51.100.2", nil)
+	if ip != "203.0.113.10" {
+		t.Fatalf("ClientIP = %q, want forwarded IP", ip)
+	}
+}
+
+func TestClientIPUsesTrustedProxyCIDRs(t *testing.T) {
+	trusted := []string{"127.0.0.1", "172.16.0.0/12"}
+	ip := ClientIP("172.20.0.4:6070", "203.0.113.11", "", trusted)
+	if ip != "203.0.113.11" {
+		t.Fatalf("trusted ClientIP = %q", ip)
+	}
+	spoofed := ClientIP("198.51.100.20:6070", "203.0.113.12", "203.0.113.13", trusted)
+	if spoofed != "198.51.100.20" {
+		t.Fatalf("untrusted ClientIP = %q, want remote addr", spoofed)
+	}
+}
+
+func TestClientIPFallsBackToRealIP(t *testing.T) {
+	ip := ClientIP("127.0.0.1:6070", "", "203.0.113.14", nil)
+	if ip != "203.0.113.14" {
+		t.Fatalf("ClientIP = %q, want real IP", ip)
+	}
+}