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/guild/guild_counts_cache.erl

@@ -0,0 +1,128 @@
+%% 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(guild_counts_cache).
+
+-export([
+    init/0,
+    update/3,
+    get/1,
+    delete/1
+]).
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+-endif.
+
+-define(TABLE, guild_counts_cache).
+
+-type guild_id() :: integer().
+
+-spec init() -> ok.
+init() ->
+    case ets:whereis(?TABLE) of
+        undefined ->
+            _ = ets:new(?TABLE, [
+                named_table,
+                public,
+                set,
+                {read_concurrency, true},
+                {write_concurrency, true}
+            ]),
+            ok;
+        _ ->
+            ok
+    end.
+
+-spec update(guild_id(), non_neg_integer(), non_neg_integer()) -> ok.
+update(GuildId, MemberCount, OnlineCount) ->
+    ensure_table(),
+    ets:insert(?TABLE, {GuildId, MemberCount, OnlineCount}),
+    ok.
+
+-spec get(guild_id()) -> {ok, non_neg_integer(), non_neg_integer()} | miss.
+get(GuildId) ->
+    case catch ets:lookup(?TABLE, GuildId) of
+        [{GuildId, MemberCount, OnlineCount}] ->
+            {ok, MemberCount, OnlineCount};
+        _ ->
+            miss
+    end.
+
+-spec delete(guild_id()) -> ok.
+delete(GuildId) ->
+    catch ets:delete(?TABLE, GuildId),
+    ok.
+
+-spec ensure_table() -> ok.
+ensure_table() ->
+    case ets:whereis(?TABLE) of
+        undefined -> init();
+        _ -> ok
+    end.
+
+-ifdef(TEST).
+
+init_creates_table_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ?assertNotEqual(undefined, ets:whereis(?TABLE)),
+    catch ets:delete(?TABLE).
+
+init_idempotent_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ok = init(),
+    ?assertNotEqual(undefined, ets:whereis(?TABLE)),
+    catch ets:delete(?TABLE).
+
+update_and_get_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ok = update(100, 50, 25),
+    ?assertEqual({ok, 50, 25}, guild_counts_cache:get(100)),
+    catch ets:delete(?TABLE).
+
+get_miss_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ?assertEqual(miss, guild_counts_cache:get(999)),
+    catch ets:delete(?TABLE).
+
+update_overwrites_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ok = update(100, 50, 25),
+    ok = update(100, 60, 30),
+    ?assertEqual({ok, 60, 30}, guild_counts_cache:get(100)),
+    catch ets:delete(?TABLE).
+
+delete_removes_entry_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ok = update(100, 50, 25),
+    ok = delete(100),
+    ?assertEqual(miss, guild_counts_cache:get(100)),
+    catch ets:delete(?TABLE).
+
+delete_nonexistent_test() ->
+    catch ets:delete(?TABLE),
+    ok = init(),
+    ok = delete(999),
+    catch ets:delete(?TABLE).
+
+-endif.