feat: add Winter user groups

- user_groups (seeded with the guest and registered groups) and the
  users_groups pivot, folded from the PHP v1.1.1 and v2.8.0 migrations
  into migration 202610020001_create_user_groups
- models.UserGroup and a User.Groups relation that is never serialized
- classes.UserGroupCodes and classes.HasGroupCode for server-side checks
  such as the site-admin group code admin
- additive only: no route, payload or existing column changes; the user
  payload still answers "groups":[]

models/user.go

@@ -30,6 +30,11 @@ type User struct {
 	CreatedIPAddress          *string        `gorm:"column:created_ip_address"`
 	LastIPAddress             *string        `gorm:"column:last_ip_address"`
 	DeletedAt                 gorm.DeletedAt `gorm:"column:deleted_at"`
+	// Groups is the PHP belongsToMany groups relation through users_groups.
+	// It is loaded only on request, written only by a caller that
+	// associates groups, and never serialized: the user API answers
+	// "groups":[] from its own payload builder.
+	Groups []UserGroup `gorm:"many2many:users_groups;joinForeignKey:user_id;joinReferences:user_group_id" json:"-"`
 }
 
 func (User) TableName() string { return "users" }

models/user_group.go

@@ -0,0 +1,29 @@
+package models
+
+import "time"
+
+// UserGroup is a row of user_groups, the PHP UserGroup model. Membership
+// lives in the users_groups pivot (User.Groups). Permissions is the raw
+// jsonable text column.
+type UserGroup struct {
+	ID          uint       `gorm:"column:id;primaryKey"`
+	Name        string     `gorm:"column:name"`
+	Code        *string    `gorm:"column:code"`
+	Description *string    `gorm:"column:description"`
+	Permissions *string    `gorm:"column:permissions"`
+	CreatedAt   *time.Time `gorm:"column:created_at"`
+	UpdatedAt   *time.Time `gorm:"column:updated_at"`
+}
+
+func (UserGroup) TableName() string { return "user_groups" }
+
+// The PHP UserGroup::GROUP_GUEST and GROUP_REGISTERED codes, seeded by the
+// create_user_groups migration.
+const (
+	GroupGuest      = "guest"
+	GroupRegistered = "registered"
+)
+
+func init() {
+	Register(UserGroup{})
+}