feat(features): add test factories to all 5 features

Adds src/__factories__/<entity>.factory.ts to auth, blog, marketing-pages,
navigation, media. Each factory uses defineFactory from @repo/core-testing
with stable date defaults (2026-01-01) so snapshot diffs reflect SUT
behavior only. Refactors mechanical inline-fixture tests to use factories.

Also adds vitest.config.ts and tsconfig path alias to @repo/media (lacked
both), and adds @repo/core-testing devDependency to @repo/media.

Spec: §5.1, §6.3

packages/navigation/src/__factories__/header.factory.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { headerFactory } from "@/__factories__/header.factory";
+
+describe("headerFactory", () => {
+  beforeEach(() => headerFactory.reset());
+
+  it("returns a Header with stable defaults", () => {
+    const h = headerFactory.build();
+    expect(h.items).toHaveLength(0);
+    expect(h.logoId).toBeUndefined();
+  });
+
+  it("applies overrides", () => {
+    const h = headerFactory.build({
+      logoId: "logo-42",
+      items: [{ label: "Home", href: "/", external: false }],
+    });
+    expect(h.logoId).toBe("logo-42");
+    expect(h.items).toHaveLength(1);
+    expect(h.items[0]?.label).toBe("Home");
+  });
+
+  it("builds multiple headers independently", () => {
+    const a = headerFactory.build({ items: [{ label: "A", href: "/a", external: false }] });
+    const b = headerFactory.build({ items: [] });
+    expect(a.items).toHaveLength(1);
+    expect(b.items).toHaveLength(0);
+  });
+});

packages/navigation/src/__factories__/header.factory.ts

@@ -0,0 +1,6 @@
+import { defineFactory } from "@repo/core-testing/factory";
+import type { Header } from "../entities/header.js";
+
+export const headerFactory = defineFactory<Header>(() => ({
+  items: [],
+}));

packages/navigation/src/__factories__/index.ts

@@ -0,0 +1 @@
+export { headerFactory } from "./header.factory.js";