feat: follow an epic with everything under it

«Seguir todo» on a ticket with subtasks (and on each epic of the Epics
page) tells you of it and of every ticket hanging from it, at any depth,
also the ones added later: their moves, assignments, comments and plan
changes, as a follower of each, plus when a ticket is placed under it.
Guests of a private project are left out. Personal, no history (Mongo
`tree_follows`).

API: followTree / unfollowTree, Issue.followingTree, PUT/DELETE
/api/issues/{issue}/follow-tree. MCP: watch_issue with tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/application/src/lib/facades/issue.facade.ts

@@ -120,6 +120,13 @@ export class IssueFacade {
     );
   }
 
+  /** Follow the ticket with everything under it (an epic), or stop. */
+  setFollowingTree(on: boolean): void {
+    this.withIssue((issueId) =>
+      this.dispatch.followTreeRequested({ issueId, on }),
+    );
+  }
+
   /** Pin the ticket for me (or unpin it): at hand on my home page and in search. */
   setPinned(pinned: boolean): void {
     this.withIssue((issueId) =>