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/application/src/lib/issues.service.ts

@@ -355,6 +355,7 @@ export class IssuesService {
       storyPoints: issue.storyPoints,
       startDate: issue.startDate,
       dueDate: issue.dueDate,
+      customFields: issue.customFields,
       parentId: issue.parentId,
       // The original's text is the content, not the type's template.
       useTemplate: false,