fix(stats): guard backfill source-less mileage

go/vehicle-gateway/cmd/stats-backfill/main_test.go

@@ -184,6 +184,71 @@ func TestWriteAggregatesClearsTargetRowsBeforeStaleCandidateUpsert(t *testing.T)
 	}
 }
 
+func TestWriteAggregatesSkipsBlankSourceBeforeClearingTarget(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("sqlmock.New() error = %v", err)
+	}
+	defer db.Close()
+
+	written, err := writeAggregates(context.Background(), db, map[string]*metricAgg{
+		"LA9GG64L7PBAF4001|2026-07-08|JT808|JT808:13307765812@": {
+			VIN:             "LA9GG64L7PBAF4001",
+			Date:            "2026-07-08",
+			Protocol:        envelope.ProtocolJT808,
+			FirstKM:         4100,
+			LatestKM:        4101,
+			Count:           1,
+			SourceKey:       "JT808:13307765812@",
+			Phone:           "13307765812",
+			SourceEndpoint:  "",
+			FirstEventTime:  time.Date(2026, 7, 8, 12, 0, 0, 0, time.FixedZone("Asia/Shanghai", 8*3600)),
+			LatestEventTime: time.Date(2026, 7, 8, 12, 0, 0, 0, time.FixedZone("Asia/Shanghai", 8*3600)),
+			QualityStatus:   stats.QualityNoPreviousBaseline,
+			QualityReason:   "missing_previous_source",
+		},
+	}, 500)
+	if err != nil {
+		t.Fatalf("writeAggregates() error = %v", err)
+	}
+	if written != 0 {
+		t.Fatalf("written = %d, want 0", written)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("sql expectations: %v", err)
+	}
+}
+
+func TestAddSamplesKeepsScanCandidatesNoPreviousBaseline(t *testing.T) {
+	loc := time.FixedZone("Asia/Shanghai", 8*3600)
+	samples, err := stats.SamplesFromEnvelope(envelope.FrameEnvelope{
+		Protocol:       envelope.ProtocolJT808,
+		VIN:            "LA9GG64L7PBAF4001",
+		Phone:          "13307765812",
+		SourceEndpoint: "115.231.168.135:20215",
+		EventTimeMS:    time.Date(2026, 7, 8, 12, 0, 0, 0, loc).UnixMilli(),
+		Fields: map[string]any{
+			"jt808.location.total_mileage_km": 4123.9,
+		},
+	}, loc)
+	if err != nil {
+		t.Fatalf("SamplesFromEnvelope() error = %v", err)
+	}
+	aggregates := map[string]*metricAgg{}
+	addSamples(aggregates, samples)
+	if len(aggregates) != 1 {
+		t.Fatalf("aggregate count = %d, want 1", len(aggregates))
+	}
+	for _, agg := range aggregates {
+		if agg.QualityStatus != stats.QualityNoPreviousBaseline {
+			t.Fatalf("quality = %q, want %q", agg.QualityStatus, stats.QualityNoPreviousBaseline)
+		}
+		if agg.QualityReason != "scan_backfill_missing_previous_source" {
+			t.Fatalf("quality reason = %q", agg.QualityReason)
+		}
+	}
+}
+
 func TestLoadConfigDefaultsBackfillMethodToLastDiff(t *testing.T) {
 	t.Setenv("BACKFILL_METHOD", "")
 	t.Setenv("BACKFILL_DATE_FROM", "2026-07-08")