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/infrastructure/src/lib/mongo/find.repositories.ts

@@ -66,6 +66,7 @@ function toProject(doc: ProjectDoc): ProjectReadModel {
     isPublic: rest.isPublic ?? false,
     acceptsProposals: rest.acceptsProposals ?? false,
     settings: rest.settings ?? defaultProjectSettings(),
+    archivedAt: rest.archivedAt ?? null,
   };
 }