local-dev: log memberships-ensured count in dev_sandbox bootstrap

Adds a single zap.Info line after the membership-insertion loop so
the boot log explicitly shows how many participants the sandbox
provisioned. The number is fixed by config (PlayerCount) but
surfacing it in the log makes troubleshooting "why is the lobby
empty" cases (typo in the email, partial failure) faster than
querying the DB.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/devsandbox/bootstrap.go

@@ -197,6 +197,10 @@ func ensureMembershipsAndDrive(ctx context.Context, svc *lobby.Service, game lob
 				return game, fmt.Errorf("dev_sandbox: insert membership %d: %w", i+1, err)
 			}
 		}
+		logger.Info("memberships ensured",
+			zap.Int("count", len(users)),
+			zap.String("game_id", game.GameID.String()),
+		)
 		next, err := svc.ReadyToStart(ctx, &caller, false, game.GameID)
 		if err != nil {
 			return game, fmt.Errorf("dev_sandbox: ready to start: %w", err)