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/api_keys.sql

@@ -13,7 +13,7 @@ SELECT * FROM team_api_keys WHERE team_id = $1 ORDER BY created_at DESC;
 SELECT k.id, k.team_id, k.name, k.key_hash, k.key_prefix, k.created_by, k.created_at, k.last_used,
        u.email AS creator_email
 FROM team_api_keys k
-JOIN users u ON u.id = k.created_by
+LEFT JOIN users u ON u.id = k.created_by
 WHERE k.team_id = $1
 ORDER BY k.created_at DESC;
 
@@ -22,3 +22,6 @@ DELETE FROM team_api_keys WHERE id = $1 AND team_id = $2;
 
 -- name: UpdateAPIKeyLastUsed :exec
 UPDATE team_api_keys SET last_used = NOW() WHERE id = $1;
+
+-- name: DeleteAPIKeysByTeam :exec
+DELETE FROM team_api_keys WHERE team_id = $1;