feat(infra): migrate Stalwart to the v0.16 config model (config.json)

v0.16 dropped TOML config. The host service now boots from a tiny config.json
that describes only the datastore (RocksDB); all other settings live in the DB
(web UI / stalwart-cli / platform-api JMAP).

- add stalwart/config.json (RocksDb datastore at /opt/stalwart/data)
- install.sh: install config.json instead of config.toml
- stalwart-mail.service: --config points at config.json
- README: document the v0.16 model + remaining DB-side config + DNS/PTR

Verified: Stalwart 0.16.8 runs on node1 with default mail listeners + the :8080
management server. config.toml retained as a reference for the DB settings.

infrastructure/production/host/stalwart/stalwart-mail.service

@@ -14,7 +14,7 @@ Type=simple
 User=stalwart
 Group=stalwart
 EnvironmentFile=/opt/stalwart/etc/stalwart.env
-ExecStart=/opt/stalwart/bin/stalwart --config /opt/stalwart/etc/config.toml
+ExecStart=/opt/stalwart/bin/stalwart --config /opt/stalwart/etc/config.json
 # Stalwart reloads its TLS certs / config on SIGHUP — used by cert-sync.
 ExecReload=/bin/kill -HUP $MAINPID
 Restart=on-failure