feat: definition of done per project

A project can ask that tickets reach a DONE status only with their
checklist ticked (settings.checklistBeforeDone, off by default; absent
when saving means unchanged). Members get a 422 telling how many steps are
left; owners and admins may still close it. Settings has the switch and
the ticket's checklist says when the project asks for it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/schema.gql

@@ -1185,6 +1185,11 @@ Statuses (board order), transitions, ticket types, areas and custom fields
 """
 type ProjectSettings {
   areas: [AreaOption!]!
+
+  """
+  Definition of done: tickets reach DONE only with their checklist ticked (owners and admins may override)
+  """
+  checklistBeforeDone: Boolean!
   customFields: [CustomFieldDefinition!]!
   issueTypes: [IssueTypeOption!]!
   statuses: [WorkflowStatus!]!
@@ -1197,6 +1202,11 @@ The whole configuration (replaces the previous one). Statuses go in board order.
 input ProjectSettingsInput {
   areas: [AreaOptionInput!]!
 
+  """
+  Definition of done: DONE only with the checklist ticked. Absent: unchanged
+  """
+  checklistBeforeDone: Boolean
+
   """Absent: unchanged. Fields cannot be removed: archive them."""
   customFields: [CustomFieldDefinitionInput!]
   issueTypes: [IssueTypeOptionInput!]!