feat: add fluxer upstream source and self-hosting documentation

- Clone of github.com/fluxerapp/fluxer (official upstream)
- SELF_HOSTING.md: full VM rebuild procedure, architecture overview,
  service reference, step-by-step setup, troubleshooting, seattle reference
- dev/.env.example: all env vars with secrets redacted and generation instructions
- dev/livekit.yaml: LiveKit config template with placeholder keys
- fluxer-seattle/: existing seattle deployment setup scripts

fluxer/fluxer_gateway/config/vm.args.template

@@ -0,0 +1,11 @@
+${FLUXER_GATEWAY_NODE_FLAG} ${FLUXER_GATEWAY_NODE_NAME}
++K true
++A30
+-env ERL_MAX_PORTS 4096
+-env ERL_PROCESSES 262144
++P 262144
++Q 65536
++S 4:4
++zdbbl 32768
+-kernel logger_level ${LOGGER_LEVEL}
+-kernel inet_backend socket