feat: complete contract-driven business development and MySQL integration

config/file.go

@@ -22,6 +22,23 @@ import (
 // field names for both formats, rejects unknown fields and accepts durations
 // such as "3s". Application-specific validation remains with the caller.
 func DecodeFile[T any](path string, defaults T) (T, error) {
+	result, err := decodeFile(path, defaults)
+	if err != nil {
+		var zero T
+		return zero, &DecodeError{cause: err}
+	}
+	return result, nil
+}
+
+// DecodeError keeps input values out of public diagnostics while preserving
+// errors.Is/As for explicitly handled underlying I/O and syntax errors.
+type DecodeError struct{ cause error }
+
+func (e *DecodeError) Error() string {
+	return "config: cannot decode file; check its path, format, field names and value types"
+}
+func (e *DecodeError) Unwrap() error { return e.cause }
+func decodeFile[T any](path string, defaults T) (T, error) {
 	var zero T
 	f, err := os.Open(path)
 	if err != nil {
@@ -64,15 +81,13 @@ func DecodeFile[T any](path string, defaults T) (T, error) {
 	if err != nil {
 		return zero, err
 	}
-	// Deep copy defaults so errors cannot partially mutate caller-owned maps.
-	original, err := json.Marshal(defaults)
+	// Clone configuration without serializing borrowed json:"-" capabilities.
+	var result T
+	original, err := cloneConfiguration(reflect.ValueOf(&defaults).Elem(), 0)
 	if err != nil {
 		return zero, err
 	}
-	var result T
-	if err = json.Unmarshal(original, &result); err != nil {
-		return zero, err
-	}
+	reflect.ValueOf(&result).Elem().Set(original)
 	dec := json.NewDecoder(bytes.NewReader(encoded))
 	dec.DisallowUnknownFields()
 	if err = dec.Decode(&result); err != nil {
@@ -115,6 +130,17 @@ func durations(v any, t reflect.Type) (any, error) {
 			}
 		}
 	}
+	if t.Kind() == reflect.Map && t.Key().Kind() == reflect.String {
+		if values, ok := v.(map[string]any); ok {
+			for key, value := range values {
+				converted, err := durations(value, t.Elem())
+				if err != nil {
+					return nil, err
+				}
+				values[key] = converted
+			}
+		}
+	}
 	if t.Kind() == reflect.Slice {
 		if items, ok := v.([]any); ok {
 			for n, item := range items {