Beta servers: testers can use beta features in servers whose owner opted in

- beta.Configure takes the opted-in servers; testers are looked up in the beta server
- beta commands registered per opted-in server and removed when it opts out
- beta.ServerEnabled for behaviour that no person triggers
- docs updated

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

schema.sql

@@ -84,6 +84,14 @@ CREATE TABLE IF NOT EXISTS beta_settings (
     updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
 
+-- Servers whose owner opted in to beta features (API migration 000022). The
+-- beta server itself is always eligible and isn't listed. The bot only reads it.
+CREATE TABLE IF NOT EXISTS beta_guilds (
+    guild_id BIGINT PRIMARY KEY,
+    enabled_by BIGINT NOT NULL,
+    enabled_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
 -- A server can have several lobbies (at most 10; the trigger enforces it, and
 -- is the same as the API's migration 000020). meeting_settings.lobby_channel_id
 -- is the old single-lobby column, no longer read.