fix(07): WR-03 add SetMaxOpenConns(2) on migration sql.DB pool

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/internal/db/migrate.go

@@ -28,6 +28,7 @@ func RunMigrations(ctx context.Context, pool *pgxpool.Pool, migrationsFS embed.F
 	if err != nil {
 		return fmt.Errorf("migrate: open sql.DB: %w", err)
 	}
+	sqlDB.SetMaxOpenConns(2) // goose runs sequentially — no need for a large pool
 	defer sqlDB.Close()
 
 	// Point goose at the embedded FS so no on-disk files are needed.