fix: validate explicit migrations and preserve large business ids

internal/service/dto/id.go

@@ -0,0 +1,38 @@
+package dto
+
+import (
+	"encoding/json"
+	"fmt"
+	"strconv"
+)
+
+// ID accepts legacy JSON integers and decimal strings. Its wire representation
+// is always a string so a browser can round-trip all uint64 values exactly.
+type ID uint64
+
+func (id ID) MarshalJSON() ([]byte, error) {
+	return json.Marshal(strconv.FormatUint(uint64(id), 10))
+}
+
+func (id *ID) UnmarshalJSON(raw []byte) error {
+	text := string(raw)
+	if len(raw) > 0 && raw[0] == '"' {
+		if err := json.Unmarshal(raw, &text); err != nil {
+			return err
+		}
+	}
+	if text == "" {
+		return fmt.Errorf("ID must be a decimal integer")
+	}
+	for _, digit := range text {
+		if digit < '0' || digit > '9' {
+			return fmt.Errorf("ID must be a decimal integer")
+		}
+	}
+	value, err := strconv.ParseUint(text, 10, 64)
+	if err != nil {
+		return fmt.Errorf("ID is outside the uint64 range")
+	}
+	*id = ID(value)
+	return nil
+}