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/events/issue.events.ts

@@ -49,6 +49,7 @@ export const issuePageEvents = eventGroup({
     }>(),
     watchRequested: type<{ issueId: string; watching: boolean }>(),
     pinRequested: type<{ issueId: string; pinned: boolean }>(),
+    followTreeRequested: type<{ issueId: string; on: boolean }>(),
     trashRequested: type<{
       issueId: string;
       trashed: boolean;