#676 Docs: add server-side TLS reference for swamp serve --cert-file/--key-file

What changed

swamp-club/swamp#675 adds --cert-file and --key-file flags to swamp serve for static TLS support. When both are provided, the server runs over wss:///https:// instead of plain ws://.

Environment variable alternatives: SWAMP_SERVE_CERT_FILE and SWAMP_SERVE_KEY_FILE.

What needs documenting

The existing content/manual/reference/tls-and-proxies.md covers client-side TLS trust stores. A new section or page should cover server-side TLS configuration:

• --cert-file and --key-file flags (both required together)
• SWAMP_SERVE_CERT_FILE / SWAMP_SERVE_KEY_FILE env vars
• PEM format requirement
• How TLS affects the WebSocket URL scheme (ws:// to wss://)
• That workers automatically derive the data-plane URL from the connect URL (wss to https)