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/src/gateway/gateway_rpc_push.erl

@@ -0,0 +1,44 @@
+%% 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(gateway_rpc_push).
+
+-export([execute_method/2]).
+
+-spec execute_method(binary(), map()) -> true.
+execute_method(<<"push.sync_user_guild_settings">>, #{
+    <<"user_id">> := UserIdBin,
+    <<"guild_id">> := GuildIdBin,
+    <<"user_guild_settings">> := UserGuildSettings
+}) ->
+    UserId = validation:snowflake_or_throw(<<"user_id">>, UserIdBin),
+    GuildId = validation:snowflake_or_throw(<<"guild_id">>, GuildIdBin),
+    push:sync_user_guild_settings(UserId, GuildId, UserGuildSettings),
+    true;
+execute_method(<<"push.sync_user_blocked_ids">>, #{
+    <<"user_id">> := UserIdBin, <<"blocked_user_ids">> := BlockedUserIds
+}) ->
+    UserId = validation:snowflake_or_throw(<<"user_id">>, UserIdBin),
+    BlockedIds = validation:snowflake_list_or_throw(<<"blocked_user_ids">>, BlockedUserIds),
+    push:sync_user_blocked_ids(UserId, BlockedIds),
+    true;
+execute_method(<<"push.invalidate_badge_count">>, #{
+    <<"user_id">> := UserIdBin
+}) ->
+    UserId = validation:snowflake_or_throw(<<"user_id">>, UserIdBin),
+    push:invalidate_user_badge_count(UserId),
+    true.