feat(redeem): support negative values for refund/deduction

Allow redeem codes with negative values to enable refund scenarios:
- Balance: negative value deducts balance (clamped to 0, never negative)
- Concurrency: negative value reduces concurrency (clamped to 0)
- Subscription: negative validity_days reduces remaining days; if
  remaining days <= 0, the subscription is canceled (set to expired)

All deductions generate standard redeem code records for audit trail.

backend/internal/handler/admin/redeem_handler_test.go

@@ -76,32 +76,38 @@ func TestCreateAndRedeem_SubscriptionRequiresGroupID(t *testing.T) {
 	assert.Equal(t, http.StatusBadRequest, code)
 }
 
-func TestCreateAndRedeem_SubscriptionRequiresPositiveValidityDays(t *testing.T) {
+func TestCreateAndRedeem_SubscriptionRequiresNonZeroValidityDays(t *testing.T) {
 	groupID := int64(5)
 	h := newCreateAndRedeemHandler()
 
-	cases := []struct {
-		name         string
-		validityDays int
-	}{
-		{"zero", 0},
-		{"negative", -1},
-	}
-
-	for _, tc := range cases {
-		t.Run(tc.name, func(t *testing.T) {
-			code := postCreateAndRedeemValidation(t, h, map[string]any{
-				"code":          "test-sub-bad-days-" + tc.name,
-				"type":          "subscription",
-				"value":         29.9,
-				"user_id":       1,
-				"group_id":      groupID,
-				"validity_days": tc.validityDays,
-			})
-
-			assert.Equal(t, http.StatusBadRequest, code)
+	// zero should be rejected
+	t.Run("zero", func(t *testing.T) {
+		code := postCreateAndRedeemValidation(t, h, map[string]any{
+			"code":          "test-sub-bad-days-zero",
+			"type":          "subscription",
+			"value":         29.9,
+			"user_id":       1,
+			"group_id":      groupID,
+			"validity_days": 0,
 		})
-	}
+
+		assert.Equal(t, http.StatusBadRequest, code)
+	})
+
+	// negative should pass validation (used for refund/reduction)
+	t.Run("negative_passes_validation", func(t *testing.T) {
+		code := postCreateAndRedeemValidation(t, h, map[string]any{
+			"code":          "test-sub-negative-days",
+			"type":          "subscription",
+			"value":         29.9,
+			"user_id":       1,
+			"group_id":      groupID,
+			"validity_days": -7,
+		})
+
+		assert.NotEqual(t, http.StatusBadRequest, code,
+			"negative validity_days should pass validation for refund")
+	})
 }
 
 func TestCreateAndRedeem_SubscriptionValidParamsPassValidation(t *testing.T) {