feat: archive a project instead of deleting it

The owner archives a finished project (archiveProject, POST
/projects/{project}/archive) and brings it back (unarchive). While
archived it stays readable, managers still read its rules and webhooks,
but no command changes it: contributing and managing checks refuse with
422, votes and public proposals too, and automations do not run. The web
shows a read-only notice on every page of it (with «Desarchivar» for the
owner), hides what edits, marks it on the home page and lists it last.

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

libs/api/tracker/domain/src/lib/read-models/project.read-model.ts

@@ -18,6 +18,8 @@ export class ProjectReadModel {
   acceptsProposals!: boolean;
   owner!: UserRefReadModel;
   members!: ProjectMemberReadModel[];
+  /** Archived since then (read-only); null while active. */
+  archivedAt!: string | null;
   /** Statuses, transitions, ticket types and areas. */
   settings!: ProjectSettings;
   createdAt!: string;