Beta: clean up servers that opted out while the bot was offline

The bot remembers where it registered beta commands (beta_registered) and
removes them from servers that are no longer in the beta at the next start.
Failed cleanups are retried; a server the bot has left is forgotten.

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

schema.sql

@@ -92,6 +92,13 @@ CREATE TABLE IF NOT EXISTS beta_guilds (
     enabled_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
 
+-- Servers where the bot registered beta commands, so it can remove them even
+-- if the server opted out while the bot was offline. Bot-owned (the API's
+-- migration 000022 creates it for a combined database).
+CREATE TABLE IF NOT EXISTS beta_registered (
+    guild_id BIGINT PRIMARY KEY
+);
+
 -- 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.