feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

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

libs/api/tracker/application/src/lib/ports/upsert-materialized-issue.repository.ts

@@ -3,13 +3,19 @@ import {
   IssueLinkReadModel,
   IssueReadModel,
   StatusCategory,
+  UserRefReadModel,
 } from '@task-stream/api/tracker/domain';
 
 export abstract class UpsertMaterializedIssueRepository {
   /** Applied only if `version` is newer than the stored one (idempotent). */
   abstract upsert(
     issue: Pick<IssueReadModel, 'id' | 'version'> &
-      Partial<Omit<IssueReadModel, 'links' | 'commentCount' | 'attachments'>>,
+      Partial<
+        Omit<
+          IssueReadModel,
+          'links' | 'commentCount' | 'attachments' | 'watchers'
+        >
+      >,
   ): Promise<void>;
   /** Adds a relation to an issue (idempotent). */
   abstract addLink(issueId: string, link: IssueLinkReadModel): Promise<void>;
@@ -26,6 +32,9 @@ export abstract class UpsertMaterializedIssueRepository {
     issueId: string,
     attachmentId: string,
   ): Promise<void>;
+  /** Adds a follower (idempotent). */
+  abstract addWatcher(issueId: string, user: UserRefReadModel): Promise<void>;
+  abstract removeWatcher(issueId: string, userId: string): Promise<void>;
   abstract setCommentCount(
     issueId: string,
     commentCount: number,