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/rebar.config

@@ -0,0 +1,62 @@
+{erl_opts, [debug_info, nowarn_deprecated_function]}.
+{deps, [
+    {cowboy, "2.14.2"},
+    {base64url, "1.0.1"},
+    {jose, "1.11.10"},
+    {ezstd, "1.1.0"},
+    {enats, "1.2.0"}
+]}.
+
+{overrides, [
+    {override, cowboy, [{deps, [cowlib, ranch]}]}
+]}.
+
+{relx, [
+    {release, {fluxer_gateway, "0.0.0"}, [
+        fluxer_gateway,
+        enats,
+        sasl
+    ]},
+    {dev_mode, true},
+    {include_erts, false},
+    {extended_start_script, true},
+    {sys_config, "./config/sys.config"},
+    {vm_args_src, "./config/vm.args.src"}
+]}.
+
+{profiles, [
+    {prod, [
+        {deps, [
+            {opentelemetry_api, "1.5.0"},
+            {opentelemetry, "1.7.0"},
+            {opentelemetry_exporter, "1.10.0"},
+            {opentelemetry_experimental, "0.5.1"},
+            {opentelemetry_api_experimental, "0.5.1"},
+            {opentelemetry_semantic_conventions, "1.27.0"}
+        ]},
+        {relx, [
+            {dev_mode, false},
+            {include_erts, true}
+        ]},
+        {erl_opts, [{d, 'HAS_OPENTELEMETRY'}]}
+    ]},
+    {dev, [
+        {erl_opts, [debug_info, nowarn_deprecated_function, {d, 'DEV_MODE'}]}
+    ]}
+]}.
+
+{shell, [
+    {config, "config/sys.config"},
+    {vm_args, "config/vm.args"},
+    {apps, [fluxer_gateway]}
+]}.
+
+{project_plugins, [erlfmt]}.
+{erlfmt, [write]}.
+
+{dialyzer, [
+    {plt_extra_apps, [
+        jose, ranch, enats
+    ]},
+    {warnings_file, "dialyzer.ignore-warnings"}
+]}.