feat: implement full CRUD for membership fee types with settings UI

- Add interval immutability and deletion prevention validations
- Add settings validation for default_membership_fee_type_id
- Create MembershipFeeSettingsLive for admin UI with form handling
- Add comprehensive test coverage (unit, integration, settings)

lib/membership_fees/membership_fee_type.ex

@@ -36,7 +36,7 @@ defmodule Mv.MembershipFees.MembershipFeeType do
   end
 
   actions do
-    defaults [:read, :destroy]
+    defaults [:read]
 
     create :create do
       primary? true
@@ -45,10 +45,69 @@ defmodule Mv.MembershipFees.MembershipFeeType do
 
     update :update do
       primary? true
+      require_atomic? false
       # Note: interval is NOT in accept list - it's immutable after creation
-      # Immutability validation will be added in a future issue
       accept [:name, :amount, :description]
     end
+
+    destroy :destroy do
+      primary? true
+      require_atomic? false
+    end
+  end
+
+  validations do
+    # Prevent interval changes after creation
+    validate fn changeset, _context ->
+      if Ash.Changeset.changing_attribute?(changeset, :interval) do
+        case changeset.data do
+          nil -> :ok  # Creating new resource, interval can be set
+          _existing -> {:error, field: :interval, message: "Interval cannot be changed after creation"}
+        end
+      else
+        :ok
+      end
+    end, on: [:update]
+
+    # Prevent deletion if assigned to members
+    validate fn changeset, _context ->
+      if changeset.action_type == :destroy do
+        require Ash.Query
+
+        member_count =
+          Mv.Membership.Member
+          |> Ash.Query.filter(membership_fee_type_id == ^changeset.data.id)
+          |> Ash.count!()
+
+        if member_count > 0 do
+          {:error, message: "Cannot delete membership fee type: #{member_count} member(s) are assigned to it"}
+        else
+          :ok
+        end
+      else
+        :ok
+      end
+    end, on: [:destroy]
+
+    # Prevent deletion if cycles exist
+    validate fn changeset, _context ->
+      if changeset.action_type == :destroy do
+        require Ash.Query
+
+        cycle_count =
+          Mv.MembershipFees.MembershipFeeCycle
+          |> Ash.Query.filter(membership_fee_type_id == ^changeset.data.id)
+          |> Ash.count!()
+
+        if cycle_count > 0 do
+          {:error, message: "Cannot delete membership fee type: #{cycle_count} cycle(s) reference it"}
+        else
+          :ok
+        end
+      else
+        :ok
+      end
+    end, on: [:destroy]
   end
 
   attributes do