fix(seed): skip seeding if database already contains markets

backend/cmd/seed/main.go

@@ -65,6 +65,15 @@ func run() error {
 		return fmt.Errorf("parse markets JSON: %w", err)
 	}
 
+	var count int
+	if err := pool.QueryRow(ctx, "SELECT count(*) FROM markets").Scan(&count); err != nil {
+		return fmt.Errorf("check market count: %w", err)
+	}
+	if count > 0 {
+		slog.Info("database already seeded, skipping", "existing", count)
+		return nil
+	}
+
 	slog.Info("seeding markets", "count", len(markets))
 
 	geoCache := make(map[string][2]float64)