Add user settings validation

backend/internal/models/settings.go

@@ -1,6 +1,42 @@
 package models
 
-type Settings struct {
-	UserID   int                    `json:"userId"`
-	Settings map[string]interface{} `json:"settings"`
+import (
+	"encoding/json"
+
+	"github.com/go-playground/validator/v10"
+)
+
+type UserSettings struct {
+	Theme    string `json:"theme" validate:"oneof=light dark"`
+	AutoSave bool   `json:"autoSave"`
+}
+
+type Settings struct {
+	UserID   int          `json:"userId" validate:"required,min=1"`
+	Settings UserSettings `json:"settings" validate:"required,dive"`
+}
+
+var validate = validator.New()
+
+func (s *Settings) Validate() error {
+	return validate.Struct(s)
+}
+
+func (s *Settings) SetDefaults(defaults UserSettings) {
+	if s.Settings.Theme == "" {
+		s.Settings.Theme = defaults.Theme
+	}
+}
+
+func (s *Settings) UnmarshalJSON(data []byte) error {
+	type Alias Settings
+	aux := &struct {
+		*Alias
+	}{
+		Alias: (*Alias)(s),
+	}
+	if err := json.Unmarshal(data, &aux); err != nil {
+		return err
+	}
+	return s.Validate()
 }