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/scripts/rebar3_wrapper.sh

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+
+# Copyright (C) 2026 Fluxer Contributors
+#
+# This file is part of Fluxer.
+#
+# Fluxer is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Fluxer is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
+
+set -euo pipefail
+
+if [ "$#" -eq 0 ]; then
+	echo "Usage: $0 <rebar3 args...>" >&2
+	exit 64
+fi
+
+ASDF_SHIMS_PATH="${ASDF_DATA_DIR:-$HOME/.asdf}/shims"
+if [ -d "$ASDF_SHIMS_PATH" ]; then
+	export PATH="$ASDF_SHIMS_PATH:$PATH"
+fi
+
+SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)
+if [ -f "$SCRIPT_DIR/../rebar.config" ]; then
+	GATEWAY_DIR=$(cd "$SCRIPT_DIR/.." && pwd)
+	REPO_ROOT=$(cd "$GATEWAY_DIR/.." && pwd)
+else
+	REPO_ROOT=$(cd "$SCRIPT_DIR/../.." && pwd)
+	GATEWAY_DIR="$REPO_ROOT/fluxer_gateway"
+fi
+
+if [ -z "${FLUXER_CONFIG:-}" ] && [ -f "$REPO_ROOT/config/config.test.json" ]; then
+	export FLUXER_CONFIG="$REPO_ROOT/config/config.test.json"
+fi
+
+should_skip_plugins=true
+for arg in "$@"; do
+	case "$arg" in
+	fmt | plugins)
+		should_skip_plugins=false
+		break
+		;;
+	esac
+done
+
+if [ "$should_skip_plugins" = true ]; then
+	export REBAR_SKIP_PROJECT_PLUGINS=1
+fi
+
+cd "$GATEWAY_DIR"
+
+exec rebar3 "$@"