✨ Implement ENABLE_REGISTRATION via user_api module ✨

- docker/rootfs/usr/local/share/prosody-plugins/mod_user_api.lua: new POST /user-api/register endpoint (JSON, validation, reserved names, per-IP rate limit, refuses when AUTH_TYPE is not internal)
- docker/rootfs/usr/local/bin/entrypoint.sh: read ENABLE_REGISTRATION, load user_api on the main host only when ENABLE_AUTH is on, warn otherwise
- docker/compose.yml: add ENABLE_REGISTRATION
- README.md: document registration

docker/compose.yml

@@ -27,6 +27,7 @@ services:
       ENABLE_XMPP_WEBSOCKET: "1"
       LOG_LEVEL: info
       # internal = local accounts only; openfire = also accept Openfire accounts
+      ENABLE_REGISTRATION: "0"
       AUTH_TYPE: internal
       OPENFIRE_AUTH_ENABLED: "0"
       OPENFIRE_AUTH_HOST: 172.17.0.1