feat: guest role for clients and outsiders

A project member can be a GUEST. In a private project a guest only sees
the tickets they take part in (opened, assigned or followed): every other
ticket answers 404, also in my work, realtime changes, blockers and
linked tickets, and project-wide views (activity, statistics, dashboard,
releases, sprints, epics, trash) answer 403. Guests open tickets without
assigning or planning them, and comment, attach, follow and vote on their
own; they change nothing else. In a public project they see what anyone
does. The web gives them only Tablero, Lista and Miembros and loads none
of the project-wide data.

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

libs/api/tracker/domain/src/lib/project/project-role.ts

@@ -4,4 +4,9 @@ export enum ProjectRole {
   MEMBER = 'MEMBER',
   /** Read-only stakeholder (PM, client...). */
   VIEWER = 'VIEWER',
+  /**
+   * Someone from outside (a client): in a private project, sees only the
+   * tickets they take part in, opens tickets and comments on theirs.
+   */
+  GUEST = 'GUEST',
 }