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]>

docs/database.md

@@ -33,7 +33,7 @@ Two things only exist in the API's migrations, because the bot never inserts int
 | Music | `music_settings`, `music_now_playing`, `music_queue`, `music_commands` | State for the dashboard's player, plus requests from the dashboard |
 | Tickets | `ticket_settings`, `ticket_categories`, `tickets`, `ticket_commands` | |
 | Security | `security_settings`, `security_trusted`, `security_quarantines`, `security_lockdowns`, `security_raids`, `security_raid_caught`, `security_snapshots`, `security_events`, `security_commands` | Settings, who is trusted, quarantines (with the roles to give back), locked channels (with the permission to restore), raids in progress, snapshots, an event history, and dashboard requests |
-| Beta | `beta_settings`, `beta_guilds` | `beta_settings` is one row: the beta server and the Beta Tester role. `beta_guilds` lists servers whose owner opted in to beta features. Written by the dashboard, read by the bot |
+| Beta | `beta_settings`, `beta_guilds` | `beta_settings` is one row: the beta server and the Beta Tester role. `beta_guilds` lists servers whose owner opted in to beta features. Both are written by the dashboard and read by the bot. `beta_registered` is the bot's own note of where it registered beta commands |
 
 Discord IDs are `BIGINT` (snowflakes fit in 64 bits). In Go they are `int64`; Discord's own API gives them to you as strings, so convert at the edge.