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/config/sys.config.template

@@ -0,0 +1,22 @@
+[
+    {kernel, [
+        {logger_level, ${LOGGER_LEVEL}},
+        {logger, [
+            {handler, default, logger_std_h, #{
+                level => ${LOGGER_LEVEL},
+                config => #{
+                    type => standard_io
+                }
+            }}
+        ]},
+        {inet_dist_use_interface, {0,0,0,0}},
+        {inet_backend, socket},
+        {inet_dist_listen_options, [{reuseaddr, true}]}
+    ]},
+    {sasl, [
+        {errlog_type, error}
+    ]},
+    {inet, [
+        {lookup, [native, file, dns]}
+    ]}
+].