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/infrastructure/src/lib/mongo/read-model.schemas.ts

@@ -199,6 +199,8 @@ class IssueDocument {
   @Prop({ type: [SchemaFactory.createForClass(AttachmentProp)] })
   attachments!: AttachmentProp[];
   @Prop({ type: [String] }) labels!: string[];
+  /** Field id → value: filtered as `customFields.<id>`. */
+  @Prop({ type: Object }) customFields!: Record<string, string>;
   @Prop({ type: Number }) version!: number;
 }
 export const IssueSchema = SchemaFactory.createForClass(IssueDocument);
@@ -413,6 +415,7 @@ export function issueFromDoc(doc: IssueDoc): IssueReadModel {
     parent: rest.parent?.id ? rest.parent : null,
     rank: rest.rank ?? implicitRank(rest.number),
     startDate: rest.startDate ?? null,
+    customFields: rest.customFields ?? {},
     sprint: rest.sprint?.id ? rest.sprint : null,
     // Links recorded before projects had several repositories name none.
     devLinks: (rest.devLinks ?? []).map((link) => ({