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/auth/src/__factories__/user.factory.test.ts

@@ -0,0 +1,26 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { userFactory } from "@/__factories__/user.factory";
+
+describe("userFactory", () => {
+  beforeEach(() => userFactory.reset());
+
+  it("returns a User with stable defaults", () => {
+    const u = userFactory.build();
+    expect(u.id).toBe("user-1");
+    expect(u.username).toBe("user1");
+    expect(u.passwordHash).toHaveLength(60);
+  });
+
+  it("applies overrides", () => {
+    const u = userFactory.build({ username: "alice" });
+    expect(u.username).toBe("alice");
+    expect(u.id).toBe("user-1");
+  });
+
+  it("increments sequence per build", () => {
+    const a = userFactory.build();
+    const b = userFactory.build();
+    expect(a.id).toBe("user-1");
+    expect(b.id).toBe("user-2");
+  });
+});