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/domain/src/lib/ports/gateways.ts

@@ -148,6 +148,8 @@ export abstract class IssuesGateway {
   abstract pins(): Observable<IssueSummary[]>;
   /** Pins the ticket for me (or unpins it): private, at most 20. */
   abstract pin(issueId: string, pinned: boolean): Observable<void>;
+  /** Follows the ticket with everything under it (or stops). */
+  abstract followTree(issueId: string, on: boolean): Observable<void>;
   /**
    * Decisions (comments marked as such) of all my projects by their text,
    * most recent first: here and not in CommentsGateway, for the palette.