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/teams.sql

@@ -74,6 +74,18 @@ WHERE t.id != '00000000-0000-0000-0000-000000000000'
 ORDER BY t.deleted_at ASC NULLS FIRST, t.created_at DESC
 LIMIT $1 OFFSET $2;
 
+-- name: ListSoleOwnedTeams :many
+-- Returns teams where the user is the owner and no other members exist.
+SELECT t.id FROM teams t
+JOIN users_teams ut ON ut.team_id = t.id
+WHERE ut.user_id = $1
+  AND ut.role = 'owner'
+  AND t.deleted_at IS NULL
+  AND NOT EXISTS (
+      SELECT 1 FROM users_teams ut2
+      WHERE ut2.team_id = t.id AND ut2.user_id <> $1
+  );
+
 -- name: CountTeamsAdmin :one
 SELECT COUNT(*)::int AS total
 FROM teams