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 ?? [],
   };
 }