feat: presence on tickets

A ticket's page beats every 10 s (touchPresence / PUT
/issues/{issue}/presence, with whether I am editing) and gets back who
else is there, editing ones first. Entries live in Redis, one hash per
ticket, and fade after 30 s without a beat; leaving the page takes me out
at once. Only for people who can read the ticket; never in the event
store. The ticket shows «Bob está editando · Ana también lo está viendo»
above its title; editing means a ticket or comment form is open or a
comment is being written.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/presenters/src/lib/http/issues.controller.ts

@@ -13,6 +13,7 @@ import {
 } from '@nestjs/common';
 import {
   ApiCreatedResponse,
+  ApiNoContentResponse,
   ApiOkResponse,
   ApiOperation,
   ApiParam,
@@ -31,6 +32,7 @@ import {
 import {
   CommentsService,
   IssuesService,
+  PresenceService,
 } from '@task-stream/api/tracker/application';
 import { IssueLinkType } from '@task-stream/api/tracker/domain';
 import { withoutNulls } from '../common/issue-input';
@@ -42,6 +44,7 @@ import {
   AddCommentDto,
   CreateIssueDto,
   IssueSearchQueryDto,
+  PresenceBodyDto,
   LinkIssueDto,
   LinkTypeParamDto,
   MyIssuesQueryDto,
@@ -60,6 +63,7 @@ import {
   EpicDto,
   IssueDto,
   IssuePageDto,
+  PresenceDto,
 } from './dto/responses.dto';
 import { viewerOf } from '../common/viewer';
 import { toBulkChange } from '../common/bulk';
@@ -85,8 +89,40 @@ export class IssuesController {
     private readonly issues: IssuesService,
     private readonly comments: CommentsService,
     private readonly refs: Refs,
+    private readonly presence: PresenceService,
   ) {}
 
+  @Put('issues/:issue/presence')
+  @ISSUE
+  @ApiOperation({
+    summary: 'I am on this ticket (heartbeat)',
+    description:
+      'Send it every ~10 s while the page is open; entries fade after 30 s without one. Answers who else is there, the ones editing first.',
+  })
+  @ApiOkResponse({ type: [PresenceDto] })
+  @ApiReadErrors()
+  async touchPresence(
+    @CurrentUser() me: User,
+    @Param('issue') issue: string,
+    @Body() body: PresenceBodyDto,
+  ) {
+    return this.presence.touch(
+      { id: me.id, name: me.name },
+      await this.refs.issueId(me.id, issue),
+      body.editing,
+    );
+  }
+
+  @Delete('issues/:issue/presence')
+  @HttpCode(204)
+  @ISSUE
+  @ApiOperation({ summary: "I left this ticket's page" })
+  @ApiNoContentResponse()
+  @ApiReadErrors()
+  async leavePresence(@CurrentUser() me: User, @Param('issue') issue: string) {
+    await this.presence.leave(me.id, await this.refs.issueId(me.id, issue));
+  }
+
   @Put('issues/:issue/watch')
   @ISSUE
   @ApiOperation({