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/Dockerfile

@@ -0,0 +1,48 @@
+FROM erlang:28-slim AS build
+
+ARG LOGGER_LEVEL=info
+
+WORKDIR /usr/src/app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+	git \
+	curl \
+	make \
+	gcc \
+	g++ \
+	libc6-dev \
+	gettext-base \
+	ca-certificates \
+	&& rm -rf /var/lib/apt/lists/*
+
+RUN curl -fsSL https://github.com/erlang/rebar3/releases/download/3.24.0/rebar3 -o /usr/local/bin/rebar3 && \
+	chmod +x /usr/local/bin/rebar3
+
+COPY rebar.config rebar.lock* ./
+RUN rebar3 compile --deps_only
+
+COPY . .
+RUN LOGGER_LEVEL=${LOGGER_LEVEL} envsubst < config/sys.config.template > config/sys.config && \
+	rebar3 as prod release
+
+FROM erlang:28-slim
+
+WORKDIR /opt/fluxer_gateway
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+	ca-certificates \
+	curl \
+	&& rm -rf /var/lib/apt/lists/*
+
+COPY --from=build /usr/src/app/_build/prod/rel/fluxer_gateway .
+COPY scripts/docker_entrypoint.sh /opt/fluxer_gateway/bin/docker_entrypoint.sh
+
+RUN chmod +x /opt/fluxer_gateway/bin/docker_entrypoint.sh && \
+	useradd -r -s /sbin/nologin -d /opt/fluxer_gateway fluxer && \
+	chown -R fluxer:fluxer /opt/fluxer_gateway
+
+USER fluxer
+
+EXPOSE 8080 8081
+
+ENTRYPOINT ["/opt/fluxer_gateway/bin/docker_entrypoint.sh"]