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/commands/watch-issue.command-handler.ts

@@ -0,0 +1,33 @@
+import { CommandHandler, ICommandHandler } from '@nestjs/cqrs';
+import { ForbiddenError } from '@task-stream/api/shared/domain';
+import { CommandResult, ack } from './command-result';
+import { WatchIssueCommand } from './watch-issue.command';
+import { WriteAccess } from './write-access';
+
+/**
+ * Following is for members, viewers included: reading a ticket is enough to
+ * want to hear about it. People only follow or unfollow for themselves.
+ */
+@CommandHandler(WatchIssueCommand)
+export class WatchIssueCommandHandler
+  implements ICommandHandler<WatchIssueCommand>
+{
+  constructor(private readonly access: WriteAccess) {}
+
+  async execute(command: WatchIssueCommand): Promise<CommandResult> {
+    const issue = await this.access.issue(command.issueId);
+    const project = await this.access.project(issue.projectId);
+    if (command.actorId !== command.userId) {
+      throw new ForbiddenError('You can only follow tickets yourself');
+    }
+    if (!project.isMember(command.userId)) {
+      throw new ForbiddenError(
+        `You are not a member of project ${project.key}`,
+      );
+    }
+    if (command.watching) issue.watch(command.actorId, command.userId);
+    else issue.unwatch(command.actorId, command.userId);
+    await issue.commitAndWait();
+    return ack(issue);
+  }
+}