Fix cascading deletion gaps for user and team cleanup

- Add ON DELETE CASCADE to users_teams, oauth_providers, admin_permissions
  and ON DELETE SET NULL (with nullable columns) to team_api_keys.created_by,
  hosts.created_by, host_tokens.created_by so HardDeleteExpiredUsers no longer
  fails with FK violations
- User account deletion now cascades to sole-owned teams via DeleteTeamInternal,
  preventing orphaned teams with live sandboxes after account removal
- ListActiveSandboxesByTeam now includes hibernated sandboxes so their disk
  snapshots are cleaned up during team deletion
- Team soft-delete now hard-deletes sandbox metric points, metric snapshots,
  API keys, and channels to prevent data accumulation on deleted teams
- Extract deleteTeamCore() to deduplicate shared logic across DeleteTeam,
  AdminDeleteTeam, and DeleteTeamInternal
- Fix ListAPIKeysByTeamWithCreator to use LEFT JOIN after created_by became
  nullable, and update handler to read pgtype.Text.String for creator_email

db/queries/channels.sql

@@ -22,6 +22,9 @@ RETURNING *;
 -- name: DeleteChannelByTeam :exec
 DELETE FROM channels WHERE id = $1 AND team_id = $2;
 
+-- name: DeleteAllChannelsByTeam :exec
+DELETE FROM channels WHERE team_id = $1;
+
 -- name: ListChannelsForEvent :many
 SELECT * FROM channels
 WHERE team_id = $1