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/graphql/project.resolver.ts

@@ -219,6 +219,27 @@ export class ProjectResolver {
     return this.projects.delete(me.id, projectId, confirmKey);
   }
 
+  @Mutation(() => CommandResultGql, {
+    description:
+      'Archive the project (owner only): everything stays readable, nothing changes until it is unarchived',
+  })
+  archiveProject(
+    @CurrentUser() me: User,
+    @Args('projectId', { type: () => ID }) projectId: string,
+  ) {
+    return this.projects.setArchived(me.id, projectId, true);
+  }
+
+  @Mutation(() => CommandResultGql, {
+    description: 'Bring an archived project back (owner only)',
+  })
+  unarchiveProject(
+    @CurrentUser() me: User,
+    @Args('projectId', { type: () => ID }) projectId: string,
+  ) {
+    return this.projects.setArchived(me.id, projectId, false);
+  }
+
   @Mutation(() => CommandResultGql, {
     description: 'Add a Pireon user to the project',
   })

libs/api/tracker/presenters/src/lib/graphql/types.ts

@@ -137,6 +137,11 @@ export class ProjectGql {
   @Field(() => UserRefGql) owner!: UserRefGql;
   @Field(() => [ProjectMemberGql]) members!: ProjectMemberGql[];
   @Field() createdAt!: string;
+  @Field(() => String, {
+    nullable: true,
+    description: 'Archived since then (read-only); null while active',
+  })
+  archivedAt!: string | null;
   @Field() updatedAt!: string;
   @Field(() => Int) version!: number;
 }