feat: custom fields per project

Text, number, date or option-list fields defined in the project settings
(archived, never removed; their type is fixed once saved). Tickets hold
them, edited in the side panel, listed under the title and filtered by
option in the list view; also over GraphQL, REST (?field=ID:value) and MCP.

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

libs/api/tracker/presenters/src/lib/graphql/enums.ts

@@ -5,6 +5,7 @@ import {
   IssueSort,
 } from '@task-stream/api/tracker/application';
 import {
+  CustomFieldType,
   IssueLinkType,
   IssuePriority,
   NotificationKind,
@@ -22,6 +23,10 @@ export enum LinkDirection {
 }
 
 registerEnumType(IssuePriority, { name: 'IssuePriority' });
+registerEnumType(CustomFieldType, {
+  name: 'CustomFieldType',
+  description: 'What a custom field holds (DATE: YYYY-MM-DD)',
+});
 registerEnumType(StatusCategory, {
   name: 'StatusCategory',
   description: