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/web/tracker/domain/src/lib/ports/gateways.ts

@@ -25,6 +25,7 @@ import {
   Release,
   ReleaseBurnup,
   SavedFilter,
+  Presence,
   MyView,
   MyViewCriteria,
   SavedFilterCriteria,
@@ -430,3 +431,9 @@ export abstract class TrackerRealtimeGateway {
    */
   abstract reconnects(): Observable<void>;
 }
+
+/** Who is on a ticket's page: a heartbeat answers who else is. */
+export abstract class PresenceGateway {
+  abstract touch(issueId: string, editing: boolean): Observable<Presence[]>;
+  abstract leave(issueId: string): Observable<void>;
+}