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

@@ -77,6 +77,7 @@ export const IssueSchema = new Schema<IssueDoc>(
     release: { _id: false, id: String, name: String },
     outwardLinks: [link],
     inwardLinks: [link],
+    watchers: [userRef],
     attachments: [
       {
         _id: false,
@@ -97,6 +98,7 @@ export const IssueSchema = new Schema<IssueDoc>(
 IssueSchema.index({ projectId: 1, status: 1, updatedAt: -1 });
 IssueSchema.index({ projectId: 1, statusCategory: 1 });
 IssueSchema.index({ 'assignee.id': 1 });
+IssueSchema.index({ 'watchers.id': 1 });
 IssueSchema.index({ 'release.id': 1 });
 IssueSchema.index({ title: 'text', description: 'text', key: 'text' });
 
@@ -179,5 +181,6 @@ export function issueFromDoc(doc: IssueDoc): IssueReadModel {
     links: [...(outwardLinks ?? []), ...(inwardLinks ?? [])],
     // Tickets projected before attachments existed have none.
     attachments: rest.attachments ?? [],
+    watchers: rest.watchers ?? [],
   };
 }

libs/api/tracker/infrastructure/src/lib/mongo/upsert-materialized.repositories.ts

@@ -18,6 +18,7 @@ import {
   ProjectReadModel,
   ReleaseReadModel,
   StatusCategory,
+  UserRefReadModel,
 } from '@task-stream/api/tracker/domain';
 import { Model } from 'mongoose';
 import {
@@ -100,15 +101,35 @@ export class MongoUpsertMaterializedIssueRepository extends UpsertMaterializedIs
     version,
     ...fields
   }: Pick<IssueReadModel, 'id' | 'version'> &
-    Partial<Omit<IssueReadModel, 'links' | 'commentCount' | 'attachments'>>) {
+    Partial<
+      Omit<
+        IssueReadModel,
+        'links' | 'commentCount' | 'attachments' | 'watchers'
+      >
+    >) {
     await upsertIfNewer(this.issues, id, version, fields, {
       outwardLinks: [],
       inwardLinks: [],
       attachments: [],
+      watchers: [],
       commentCount: 0,
     });
   }
 
+  async addWatcher(issueId: string, user: UserRefReadModel) {
+    await this.issues.updateOne(
+      { _id: issueId, 'watchers.id': { $ne: user.id } },
+      { $push: { watchers: user } },
+    );
+  }
+
+  async removeWatcher(issueId: string, userId: string) {
+    await this.issues.updateOne(
+      { _id: issueId },
+      { $pull: { watchers: { id: userId } } },
+    );
+  }
+
   async addAttachment(issueId: string, attachment: AttachmentReadModel) {
     await this.issues.updateOne(
       { _id: issueId, 'attachments.id': { $ne: attachment.id } },