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/presenters/src/lib/http/issues.controller.ts

@@ -77,6 +77,36 @@ export class IssuesController {
     private readonly refs: Refs,
   ) {}
 
+  @Put('issues/:issue/watch')
+  @ISSUE
+  @ApiOperation({
+    summary: 'Follow a ticket (members, viewers included)',
+    description:
+      'Followers are told when it moves, is reassigned or gets a comment. Commenting follows it too.',
+  })
+  @ApiOkResponse({ type: CommandResultDto })
+  @ApiCommandErrors()
+  async watch(@CurrentUser() me: User, @Param('issue') issue: string) {
+    return this.issues.watch(
+      me.id,
+      await this.refs.issueId(me.id, issue),
+      true,
+    );
+  }
+
+  @Delete('issues/:issue/watch')
+  @ISSUE
+  @ApiOperation({ summary: 'Stop following a ticket' })
+  @ApiOkResponse({ type: CommandResultDto })
+  @ApiCommandErrors()
+  async unwatch(@CurrentUser() me: User, @Param('issue') issue: string) {
+    return this.issues.watch(
+      me.id,
+      await this.refs.issueId(me.id, issue),
+      false,
+    );
+  }
+
   @Get('me/issues')
   @ApiOperation({
     summary: 'My work: tickets across all my projects',