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/presenters/src/lib/http/dto/responses.dto.ts

@@ -128,6 +128,13 @@ export class ProjectDto {
   @ApiProperty({ type: ProjectSettingsDto })
   settings!: ProjectSettingsDto;
   @ApiProperty({ format: 'date-time' }) createdAt!: string;
+  @ApiProperty({
+    type: String,
+    nullable: true,
+    format: 'date-time',
+    description: 'Archived since then (read-only); null while active',
+  })
+  archivedAt!: string | null;
   @ApiProperty({ format: 'date-time' }) updatedAt!: string;
   @ApiProperty({ description: 'Version of the aggregate (event count)' })
   version!: number;