feat: public projects, project deletion and sagas

Public projects:
- Project.isPublic (create/update, owners and admins). Anyone can read a
  public project without a token: project, board, tickets, comments,
  history, releases, activity, tracking and statistics, over GraphQL and
  REST. Writes stay for members (401 anonymous, 403 outsiders).
- @OptionalAuth() / @ApiOptionalPireonAuth(): anonymous callers allowed,
  invalid tokens still rejected. ReadAccess lets anonymous viewers read
  public projects only; linked tickets are filtered by visibility.
- publicProjects (GraphQL) and GET /api/explore/projects (REST).
- Web: «Explorar» page, project pages open without a session (read-only
  notice, sign-in hints), public badge, visibility switch in settings,
  no realtime subscriptions without a session.

Project deletion:
- Owner only, confirming the key (deleteProject / DELETE
  /api/projects/{project}?confirm=KEY). The key is freed; the read model
  drops the project and members are notified (web goes home).

Sagas (run by the EventsBridge after projections, durable and ordered):
- ProjectDeletionSaga: project deleted → delete its tickets and releases;
  ticket deleted → purge its comments and drop links from other tickets.
- ReleaseArchivedSaga: unfinished tickets leave an archived release.
- UserDeactivatedSaga: the Pireon webhook records UserDeactivatedEvent in
  the event store (EventRecorder); the tracker unassigns that user's open
  tickets as the system actor.
- Internal idempotent commands; domain refusals are logged and skipped,
  concurrency conflicts retried. Affected streams come from the event store
  (TrackerStreamsRepository).

Tests: domain lifecycle specs, saga specs run like the bridge, facade
tests, e2e for public access, cascade deletion, archived releases and
deactivation webhook. Docs, READMEs, schema.gql/openapi.json and Word/PDF
updated.

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

libs/api/tracker/application/src/lib/event-handlers/activity.event-handler.ts

@@ -22,6 +22,8 @@ import {
   ReleaseUpdatedEvent,
   TrackerEvent,
   DEFAULT_PROJECT_SETTINGS,
+  SYSTEM_ACTOR_ID,
+  SYSTEM_ACTOR_NAME,
   ProjectSettingsChangedEvent,
   ProjectWorkflow,
 } from '@task-stream/api/tracker/domain';
@@ -80,12 +82,16 @@ export class ActivityEventHandler implements IEventHandler<TrackerEvent> {
       ? await this.findReleases.findById(releaseId)
       : null;
     const project = await this.findProjects.findById(event.projectId);
+    // The project was deleted (cascade in progress): no feed to write to.
+    if (!project) return;
     const mentioned = [
       event.actorId,
       (event as { userId?: string }).userId,
       (event as { assigneeId?: string }).assigneeId,
     ];
     const users = await this.users.refs(mentioned);
+    const actorOf = (id: string) =>
+      id === SYSTEM_ACTOR_ID ? { id, name: SYSTEM_ACTOR_NAME } : users.of(id);
 
     const activity: ActivityReadModel = {
       id: `${event.aggregateId}:${versionOf(event)}`,
@@ -93,9 +99,9 @@ export class ActivityEventHandler implements IEventHandler<TrackerEvent> {
       projectId: event.projectId,
       issueId,
       issueKey,
-      actor: users.of(event.actorId),
+      actor: actorOf(event.actorId),
       summary: describeEvent(event, {
-        actor: users.of(event.actorId).name,
+        actor: actorOf(event.actorId).name,
         user: (id) => users.ofNullable(id)?.name ?? '?',
         issueKey: issueKey ?? '',
         projectKey: project?.key ?? '',