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

pkg/db/channels.sql.go

@@ -11,6 +11,15 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const deleteAllChannelsByTeam = `-- name: DeleteAllChannelsByTeam :exec
+DELETE FROM channels WHERE team_id = $1
+`
+
+func (q *Queries) DeleteAllChannelsByTeam(ctx context.Context, teamID pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, deleteAllChannelsByTeam, teamID)
+	return err
+}
+
 const deleteChannelByTeam = `-- name: DeleteChannelByTeam :exec
 DELETE FROM channels WHERE id = $1 AND team_id = $2
 `