Migrate database and models for study-group-based authorization

app/models/study_group_membership.rb

@@ -4,5 +4,11 @@ class StudyGroupMembership < ApplicationRecord
   belongs_to :user, polymorphic: true
   belongs_to :study_group
 
+  enum role: {
+    learner: 0,
+    teacher: 1,
+  }, _default: :learner, _prefix: true
+
+  validates :role, presence: true
   validates :user_id, uniqueness: {scope: %i[user_type study_group_id]}
 end