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

internal/api/handlers_apikeys.go

@@ -63,7 +63,7 @@ func apiKeyWithCreatorToResponse(k db.ListAPIKeysByTeamWithCreatorRow) apiKeyRes
 		Name:         k.Name,
 		KeyPrefix:    k.KeyPrefix,
 		CreatedBy:    id.FormatUserID(k.CreatedBy),
-		CreatorEmail: k.CreatorEmail,
+		CreatorEmail: k.CreatorEmail.String,
 	}
 	if k.CreatedAt.Valid {
 		resp.CreatedAt = k.CreatedAt.Time.Format(time.RFC3339)