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_relay/rebar.config

@@ -0,0 +1,37 @@
+{erl_opts, [debug_info, {parse_transform, lager_transform}]}.
+
+{deps, [
+    {cowboy, "2.12.0"},
+    {gun, "2.1.0"},
+    {lager, "3.9.2"}
+]}.
+
+{relx, [
+    {release, {fluxer_relay, "0.1.0"}, [fluxer_relay, sasl, crypto, inets, ssl, public_key]},
+    {mode, dev},
+    {include_erts, false},
+    {extended_start_script, true},
+    {sys_config_src, "config/sys.config.src"},
+    {vm_args_src, "config/vm.args.src"}
+]}.
+
+{profiles, [
+    {prod, [
+        {relx, [
+            {mode, prod},
+            {include_erts, true}
+        ]}
+    ]}
+]}.
+
+{shell, [
+    {config, "config/sys.config.src"},
+    {apps, [fluxer_relay]}
+]}.
+
+{project_plugins, [erlfmt]}.
+{erlfmt, [write]}.
+
+{dialyzer, [
+    {plt_extra_apps, [inets, ssl, public_key]}
+]}.