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/application/src/lib/events/presence.events.ts

@@ -0,0 +1,19 @@
+import { type } from '@ngrx/signals';
+import { eventGroup } from '@ngrx/signals/events';
+import { Presence } from '@task-stream/web/tracker/domain';
+
+export const presencePageEvents = eventGroup({
+  source: 'Issue Presence',
+  events: {
+    opened: type<string>(),
+    editingChanged: type<boolean>(),
+    left: type<void>(),
+  },
+});
+
+export const presenceApiEvents = eventGroup({
+  source: 'Presence API',
+  events: {
+    seen: type<Presence[]>(),
+  },
+});