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.

.env.example

@@ -30,3 +30,4 @@ WARPBOX_USER_STORAGE_BACKEND=local
 WARPBOX_READ_TIMEOUT=15s
 WARPBOX_WRITE_TIMEOUT=60s
 WARPBOX_IDLE_TIMEOUT=120s
+WARPBOX_TRUSTED_PROXIES=