perf(gr26): batch signal inserts into a single round trip

gr26/service/signal.go

@@ -14,9 +14,12 @@ import (
 
 // produced_at is MATERIALIZED from timestamp; created_at defaults to insert
 // wall clock and acts as the ReplacingMergeTree version. Both omitted here.
-const insertSignalSQL = `INSERT INTO signal (id, timestamp, vehicle_id, name, value, raw_value) VALUES (?, ?, ?, ?, ?, ?)`
+const insertSignalSQL = `INSERT INTO signal (id, timestamp, vehicle_id, name, value, raw_value)`
 
 func CreateSignals(signals []mapache.Signal) error {
+	if len(signals) == 0 {
+		return nil
+	}
 	// Stamp CreatedAt locally before any MQTT publish or CH insert so
 	// downstream consumers (live cache eviction, SSE Last-Event-ID) have
 	// a reliable wall-clock anchor independent of producer/CAN-frame clock
@@ -39,13 +42,18 @@ func CreateSignals(signals []mapache.Signal) error {
 	if !config.ClickhouseEnabled() {
 		return nil
 	}
-	ctx := database.InsertCtx(context.Background())
+	// One buffered block insert per call — a single round trip regardless of
+	// signal count, which matters with CH ~25ms away from foundry. async_insert
+	// (InsertCtx) doesn't apply to native block inserts, so plain ctx here.
+	batch, err := database.Conn.PrepareBatch(context.Background(), insertSignalSQL)
+	if err != nil {
+		return err
+	}
+	defer batch.Close()
 	for _, s := range signals {
-		if err := database.Conn.Exec(ctx, insertSignalSQL,
-			s.ID, int64(s.Timestamp), s.VehicleID, s.Name, s.Value, int64(s.RawValue),
-		); err != nil {
+		if err := batch.Append(s.ID, int64(s.Timestamp), s.VehicleID, s.Name, s.Value, int64(s.RawValue)); err != nil {
 			return err
 		}
 	}
-	return nil
+	return batch.Send()
 }