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/src/relay/fluxer_relay_env.erl

@@ -0,0 +1,82 @@
+%% 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/>.
+
+-module(fluxer_relay_env).
+
+-export([load/0, get/1, get_optional/1, get_map/0, patch/1, update/1]).
+
+-define(CONFIG_TERM_KEY, {fluxer_relay, runtime_config}).
+
+-type config() :: map().
+
+-spec load() -> config().
+load() ->
+    Config = build_config(),
+    apply_system_config(Config),
+    set_config(Config).
+
+-spec get(atom()) -> term().
+get(Key) when is_atom(Key) ->
+    Map = get_map(),
+    maps:get(Key, Map, undefined).
+
+-spec get_optional(atom()) -> term().
+get_optional(Key) when is_atom(Key) ->
+    ?MODULE:get(Key).
+
+-spec get_map() -> config().
+get_map() ->
+    ensure_loaded().
+
+-spec patch(map()) -> config().
+patch(Patch) when is_map(Patch) ->
+    Map = get_map(),
+    set_config(maps:merge(Map, Patch)).
+
+-spec update(fun((config()) -> config())) -> config().
+update(Fun) when is_function(Fun, 1) ->
+    Map = get_map(),
+    set_config(Fun(Map)).
+
+-spec set_config(config()) -> config().
+set_config(Config) when is_map(Config) ->
+    persistent_term:put(?CONFIG_TERM_KEY, Config),
+    Config.
+
+-spec ensure_loaded() -> config().
+ensure_loaded() ->
+    case persistent_term:get(?CONFIG_TERM_KEY, undefined) of
+        Map when is_map(Map) ->
+            Map;
+        _ ->
+            load()
+    end.
+
+-spec build_config() -> config().
+build_config() ->
+    fluxer_relay_config:load().
+
+-spec apply_system_config(config()) -> ok.
+apply_system_config(Config) ->
+    apply_logger_config(Config),
+    ok.
+
+-spec apply_logger_config(config()) -> ok.
+apply_logger_config(Config) ->
+    LoggerLevel = maps:get(logger_level, Config, info),
+    lager:set_loglevel(lager_console_backend, LoggerLevel),
+    ok.