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/web/tracker/domain/src/lib/models/models.ts

@@ -31,6 +31,8 @@ export interface Project {
   owner: UserRef;
   members: ProjectMember[];
   settings: ProjectSettings;
+  /** Archived since then (read-only); null while active. */
+  archivedAt: string | null;
   updatedAt: string;
   version: number;
 }