refactor(features): split entities into models/ + errors/ subdirs

All 5 features (auth, blog, marketing-pages, navigation; media has no
entities yet) now follow Lazar's pattern:
- entities/<x>.ts → entities/models/<x>.ts
- entities/errors.ts → entities/errors/<domain>.ts + errors/common.ts

Updates all import paths across factories, contracts, tests, use cases,
controllers, repositories, integrations, and src/index.ts exports.

navigation divergence: had no errors.ts; errors/header.ts +
errors/common.ts added as new forward-looking stubs.

Refactor log: docs/superpowers/refactor-logs/2026-05-05-lazar-pattern-conformance.md
Spec: §5, §9.3

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/marketing-pages/src/__contracts__/pages-repository.contract.ts

@@ -1,7 +1,7 @@
 import { it, expect, beforeEach } from "vitest";
 import { defineContractSuite } from "@repo/core-testing/contract";
 import type { IPagesRepository } from "../application/repositories/pages-repository.interface.js";
-import type { Page } from "../entities/page.js";
+import type { Page } from "../entities/models/page.js";
 
 const SEED_DATE = new Date("2026-01-01T00:00:00.000Z");