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;

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) =>

libs/web/tracker/application/src/lib/stores/issue.store.ts

@@ -379,6 +379,20 @@ export const IssueStore = signalStore(
                   ),
             ),
           ),
+          events
+            .on(issuePageEvents.followTreeRequested)
+            .pipe(
+              mergeMap(({ payload }) =>
+                runCommand(
+                  issues.followTree(payload.issueId, payload.on),
+                  payload.on
+                    ? 'Ahora sigues este ticket con todo lo que cuelga de él'
+                    : 'Ya no sigues lo que cuelga de este ticket',
+                  done,
+                  failed,
+                ),
+              ),
+            ),
           events
             .on(issuePageEvents.pinRequested)
             .pipe(