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/graphql/issue.resolver.ts

@@ -17,6 +17,7 @@ import {
   BlockersService,
   CommentsService,
   IssuesService,
+  PresenceService,
   WorkflowsService,
 } from '@task-stream/api/tracker/application';
 import {
@@ -48,6 +49,7 @@ import {
   IssueGql,
   IssuePageGql,
   IssueParentGql,
+  PresenceGql,
 } from './types';
 
 @Resolver(() => IssueGql)
@@ -57,8 +59,30 @@ export class IssueResolver {
     private readonly commentsService: CommentsService,
     private readonly workflows: WorkflowsService,
     private readonly blockers: BlockersService,
+    private readonly presence: PresenceService,
   ) {}
 
+  @Mutation(() => [PresenceGql], {
+    description:
+      'I am on this ticket (send every ~10 s while the page is open): answers who else is, the ones editing first',
+  })
+  touchPresence(
+    @CurrentUser() me: User,
+    @Args('issueId', { type: () => ID }) issueId: string,
+    @Args('editing', { defaultValue: false }) editing: boolean,
+  ) {
+    return this.presence.touch({ id: me.id, name: me.name }, issueId, editing);
+  }
+
+  @Mutation(() => Boolean, { description: "I left this ticket's page" })
+  async leavePresence(
+    @CurrentUser() me: User,
+    @Args('issueId', { type: () => ID }) issueId: string,
+  ) {
+    await this.presence.leave(me.id, issueId);
+    return true;
+  }
+
   @OptionalAuth()
   @Query(() => IssueGql)
   issue(