fix platform workflow integrity and permissions

backend/api/internal/logic/platform/fin/fin.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
+	"time"
 
 	"git.apinb.com/bsm-sdk/core/errcode"
 	"git.apinb.com/bsm-sdk/core/infra"
@@ -40,6 +41,9 @@ func rewriteSettlementSubject(ctx *gin.Context) error {
 	if err := ctx.ShouldBindJSON(&input); err != nil {
 		return err
 	}
+	if !validSettlementPeriod(input["period_start"], input["period_end"]) {
+		return errors.New("invalid settlement period")
+	}
 	subjectType, _ := input["subject_type"].(string)
 	identity, _ := input["subject_identity"].(string)
 	var model any
@@ -66,3 +70,14 @@ func rewriteSettlementSubject(ctx *gin.Context) error {
 	ctx.Request.Body = io.NopCloser(bytes.NewReader(encoded))
 	return nil
 }
+
+func validSettlementPeriod(startValue, endValue any) bool {
+	startText, startOK := startValue.(string)
+	endText, endOK := endValue.(string)
+	if !startOK || !endOK {
+		return false
+	}
+	start, startErr := time.Parse(time.RFC3339, startText)
+	end, endErr := time.Parse(time.RFC3339, endText)
+	return startErr == nil && endErr == nil && end.After(start)
+}

backend/api/internal/logic/platform/fin/fin_test.go

@@ -0,0 +1,12 @@
+package fin
+
+import "testing"
+
+func TestSettlementPeriodMustMoveForward(t *testing.T) {
+	if !validSettlementPeriod("2026-01-01T00:00:00Z", "2026-02-01T00:00:00Z") {
+		t.Fatal("valid settlement period was rejected")
+	}
+	if validSettlementPeriod("2026-02-01T00:00:00Z", "2026-01-01T00:00:00Z") {
+		t.Fatal("reversed settlement period was accepted")
+	}
+}