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/read-models/issue.read-model.ts

@@ -77,6 +77,8 @@ export class IssueReadModel {
   storyPoints!: number | null;
   startDate!: string | null;
   dueDate!: string | null;
+  /** The project's custom fields: field id → value (only those with one). */
+  customFields!: Record<string, string>;
   links!: IssueLinkReadModel[];
   attachments!: AttachmentReadModel[];
   /** People following the ticket. */