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/domain/src/lib/events/issue-updated.event.ts

@@ -1,5 +1,5 @@
 import { AutowiredEvent } from '@task-stream/api/shared/domain';
-import type { IssueDetails } from '../issue/issue';
+import type { IssueChanges } from '../issue/issue';
 import { TrackerEvent } from './tracker.event';
 
 /** Editable fields of a ticket changed. */
@@ -9,7 +9,7 @@ export class IssueUpdatedEvent extends TrackerEvent {
     actorId: string,
     issueId: string,
     projectId: string,
-    readonly changes: IssueDetails,
+    readonly changes: IssueChanges,
   ) {
     super(actorId, issueId, projectId);
   }