Add proxy mode for virtual printer (cross-LAN slicer support)

Enable Bambu Studio on a remote network to print through BamBuddy
acting as a TLS-terminating proxy for both MQTT and FTP connections.

- Add TLSProxy base class and FTPTLSProxy with PASV response rewriting,
  EPSV→PASV translation, PROT P/C tracking, and one-shot data proxies
- Add SlicerProxyManager to coordinate per-slicer MQTT + FTP proxy pairs
- Support additional SAN IPs in certificate generation for proxy mode
- Broadcast SSDP on LAN B so slicers discover the proxy as a printer
- Narrow FTP passive port range to 50000-50100 with retry logic
- Expose proxy ports (8883, 9990, 50000-50100) in Dockerfile
- Document passive port range in docker-compose.yml

Dockerfile

@@ -47,6 +47,9 @@ ENV LOG_DIR=/app/logs
 ENV PORT=8000
 
 EXPOSE 8000
+EXPOSE 8883
+EXPOSE 9990
+EXPOSE 50000-50100
 
 # Health check (uses PORT env var via shell)
 HEALTHCHECK --interval=30s --timeout=10s --start-period=10s --retries=3 \