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/.devcontainer/Dockerfile

@@ -0,0 +1,40 @@
+# Language runtimes (Node.js, Go, Rust, Python) are installed via devcontainer
+# features. This Dockerfile handles Erlang/OTP (no feature available) and
+# tools like Caddy, process-compose, rebar3, uv, ffmpeg, and exiftool.
+
+FROM erlang:28-slim AS erlang
+
+FROM mcr.microsoft.com/devcontainers/base:debian-13
+
+ARG DEBIAN_FRONTEND=noninteractive
+ARG REBAR3_VERSION=3.24.0
+ARG PROCESS_COMPOSE_VERSION=1.90.0
+
+# Both erlang:28-slim and debian-13 are Trixie-based, so OpenSSL versions match.
+COPY --from=erlang /usr/local/lib/erlang /usr/local/lib/erlang
+RUN ln -sf /usr/local/lib/erlang/bin/* /usr/local/bin/
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+	libncurses6 libsctp1 \
+	build-essential pkg-config \
+	ffmpeg libimage-exiftool-perl \
+	sqlite3 libsqlite3-dev \
+	libssl-dev openssl \
+	gettext-base lsof iproute2 \
+	&& rm -rf /var/lib/apt/lists/*
+
+RUN curl -fsSL "https://github.com/erlang/rebar3/releases/download/${REBAR3_VERSION}/rebar3" \
+	-o /usr/local/bin/rebar3 \
+	&& chmod +x /usr/local/bin/rebar3
+
+RUN curl -fsSL "https://caddyserver.com/api/download?os=linux&arch=amd64" \
+	-o /usr/local/bin/caddy \
+	&& chmod +x /usr/local/bin/caddy
+
+RUN curl -fsSL "https://github.com/F1bonacc1/process-compose/releases/download/v${PROCESS_COMPOSE_VERSION}/process-compose_linux_amd64.tar.gz" \
+	| tar xz -C /usr/local/bin process-compose \
+	&& chmod +x /usr/local/bin/process-compose
+
+RUN curl -fsSL "https://github.com/astral-sh/uv/releases/latest/download/uv-x86_64-unknown-linux-gnu.tar.gz" \
+	| tar xz --strip-components=1 -C /usr/local/bin \
+	&& chmod +x /usr/local/bin/uv /usr/local/bin/uvx