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/command-handlers.ts

@@ -27,6 +27,7 @@ import { DetachIssueFromReleaseCommandHandler } from './detach-issue-from-releas
 import { UnassignIssueCommandHandler } from './unassign-issue.command-handler';
 import { AttachFileCommandHandler } from './attach-file.command-handler';
 import { DetachFileCommandHandler } from './detach-file.command-handler';
+import { WatchIssueCommandHandler } from './watch-issue.command-handler';
 
 export const COMMAND_HANDLERS = [
   CreateProjectCommandHandler,
@@ -59,4 +60,5 @@ export const COMMAND_HANDLERS = [
   UnassignIssueCommandHandler,
   AttachFileCommandHandler,
   DetachFileCommandHandler,
+  WatchIssueCommandHandler,
 ];

libs/api/tracker/application/src/lib/commands/commands.spec.ts

@@ -40,6 +40,7 @@ import { AddProjectMemberCommand } from './add-project-member.command';
 import { AssignIssueCommand } from './assign-issue.command';
 import { AttachFileCommand } from './attach-file.command';
 import { DetachFileCommand } from './detach-file.command';
+import { WatchIssueCommand } from './watch-issue.command';
 import { ChangeIssueStatusCommand } from './change-issue-status.command';
 import { COMMAND_HANDLERS } from './command-handlers';
 import { CommandResult } from './command-result';
@@ -258,6 +259,28 @@ describe('tracker commands (event sourced)', () => {
     });
   });
 
+  it('lets any member, viewers included, follow a ticket for themselves', async () => {
+    const { run, rehydrator } = await setup();
+    const projectId = await projectWithTeam(run);
+    const issue = await run(
+      new CreateIssueCommand('qa', projectId, IssueType.BUG, 'Crash', {}),
+    );
+
+    await run(new WatchIssueCommand('pm', issue.id, 'pm'));
+    await run(new WatchIssueCommand('dev', issue.id, 'dev'));
+    await run(new WatchIssueCommand('dev', issue.id, 'dev', false));
+
+    expect((await rehydrator.rehydrate(issue.id, Issue)).watchers).toEqual([
+      'pm',
+    ]);
+    await expect(
+      run(new WatchIssueCommand('dev', issue.id, 'pm', false)),
+    ).rejects.toThrow(ForbiddenError);
+    await expect(
+      run(new WatchIssueCommand('gone', issue.id, 'gone')),
+    ).rejects.toThrow(ForbiddenError);
+  });
+
   it('does not let viewers modify tickets', async () => {
     const { run } = await setup();
     const projectId = await projectWithTeam(run);

libs/api/tracker/application/src/lib/commands/index.ts

@@ -28,3 +28,4 @@ export * from './detach-issue-from-release.command';
 export * from './unassign-issue.command';
 export * from './attach-file.command';
 export * from './detach-file.command';
+export * from './watch-issue.command';

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);
+  }
+}

libs/api/tracker/application/src/lib/commands/watch-issue.command.ts

@@ -0,0 +1,14 @@
+import { Command } from '@nestjs/cqrs';
+import { CommandResult } from './command-result';
+
+/** `userId` starts (`watching`) or stops following a ticket. */
+export class WatchIssueCommand extends Command<CommandResult> {
+  constructor(
+    public readonly actorId: string,
+    public readonly issueId: string,
+    public readonly userId: string,
+    public readonly watching = true,
+  ) {
+    super();
+  }
+}