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/dto/responses.dto.ts

@@ -764,6 +764,14 @@ export class ProjectStatsDto {
   byAssignee!: CountByAssigneeDto[];
 }
 
+export class PresenceDto {
+  @ApiProperty({ type: UserRefDto }) user!: UserRefDto;
+  @ApiProperty({ description: 'Editing the ticket or writing a comment' })
+  editing!: boolean;
+  @ApiProperty({ format: 'date-time', description: 'Last heartbeat' })
+  at!: string;
+}
+
 export class ActivityDto {
   @ApiProperty() id!: string;
   @ApiProperty({ example: 'IssueStatusChanged' }) type!: string;