fix(10.1-03): fill numeric model fields from JSON numbers

- lagoon.Fill converts a json.Number (from a UseNumber decoder, as cabana's
  save path uses) into integer, unsigned and float fields; a fraction or an
  overflow is an error
- before this, saving a type: number field into an *int column was a 500
- README documents the conversion

modules/lagoon/fill_test.go

@@ -2,6 +2,7 @@ package lagoon
 
 import (
 	"bytes"
+	"encoding/json"
 	"log/slog"
 	"strings"
 	"testing"
@@ -160,3 +161,35 @@ func TestFillEncryptedTakesPlaintext(t *testing.T) {
 		t.Fatalf("nil must clear: api_key=%q token=%v", row.APIKey.Reveal(), row.Token)
 	}
 }
+
+// TestFillJSONNumber covers request bodies decoded with UseNumber: a
+// json.Number fills integer, unsigned and float fields (plain or pointer), a
+// string field keeps the number's text, and a fraction or an overflow into an
+// integer field is an error rather than a silent truncation.
+func TestFillJSONNumber(t *testing.T) {
+	type numbers struct {
+		Year  *int    `gorm:"column:year"`
+		Count uint8   `gorm:"column:count"`
+		Price float64 `gorm:"column:price"`
+		Code  string  `gorm:"column:code"`
+	}
+	allowed := []string{"year", "count", "price", "code"}
+	var row numbers
+	err := Fill(&row, allowed, map[string]any{
+		"year":  json.Number("1977"),
+		"count": json.Number("200"),
+		"price": json.Number("12.5"),
+		"code":  json.Number("0042"),
+	}, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if row.Year == nil || *row.Year != 1977 || row.Count != 200 || row.Price != 12.5 || row.Code != "0042" {
+		t.Fatalf("row = %+v (year %v)", row, row.Year)
+	}
+	for key, value := range map[string]json.Number{"year": "1977.5", "count": "256"} {
+		if err := Fill(&row, allowed, map[string]any{key: value}, true); err == nil {
+			t.Fatalf("%s = %s filled without an error", key, value)
+		}
+	}
+}