chore(server): remove useless free subscription enum (#6634)

packages/backend/server/src/plugins/payment/resolver.ts

@@ -56,7 +56,10 @@ export class UserSubscriptionType implements Partial<UserSubscription> {
   @Field({ name: 'id' })
   stripeSubscriptionId!: string;
 
-  @Field(() => SubscriptionPlan)
+  @Field(() => SubscriptionPlan, {
+    description:
+      "The 'Free' plan just exists to be a placeholder and for the type convenience of frontend.\nThere won't actually be a subscription with plan 'Free'",
+  })
   plan!: SubscriptionPlan;
 
   @Field(() => SubscriptionRecurring)

packages/backend/server/src/schema.gql

@@ -443,6 +443,11 @@ type UserSubscription {
   end: DateTime!
   id: String!
   nextBillAt: DateTime
+
+  """
+  The 'Free' plan just exists to be a placeholder and for the type convenience of frontend.
+  There won't actually be a subscription with plan 'Free'
+  """
   plan: SubscriptionPlan!
   recurring: SubscriptionRecurring!
   start: DateTime!