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/models/models.ts

@@ -197,6 +197,15 @@ export interface Issue extends IssueSummary {
   createdAt: string;
 }
 
+/** Someone else on a ticket's page right now. */
+export interface Presence {
+  user: UserRef;
+  /** Editing the ticket or writing a comment. */
+  editing: boolean;
+  /** Last heartbeat. */
+  at: string;
+}
+
 /** A commit, branch or pull request that mentions a ticket. */
 export interface DevLink {
   kind: 'COMMIT' | 'BRANCH' | 'PULL_REQUEST';