refactor(blog): factory-style use cases + per-use-case controllers + getArticleBySlug

- Use cases (create-article, get-articles, get-article-by-slug NEW) → factory functions
- Controllers split: articles.controller.ts → 3 single-responsibility files
- DI module wires factories with .toDynamicValue()
- tRPC router resolves controllers via container

Refactor log: §2, §3, §4.1, §4.2, §5.1
Spec: §6.2

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/blog/src/application/use-cases/get-article-by-slug.use-case.test.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from "vitest";
+import { getArticleBySlugUseCase } from "@/application/use-cases/get-article-by-slug.use-case";
+import { MockArticlesRepository } from "@/infrastructure/repositories/articles.repository.mock";
+import { ArticleNotFoundError } from "@/entities/errors/article";
+import { articleFactory } from "@/__factories__/article.factory";
+
+describe("getArticleBySlugUseCase", () => {
+  it("returns the article when slug exists", async () => {
+    const repo = new MockArticlesRepository();
+    const seed = articleFactory.build({ slug: "test-slug" });
+    await repo.createArticle(seed);
+
+    const useCase = getArticleBySlugUseCase(repo);
+    const result = await useCase({ slug: "test-slug" });
+
+    expect(result?.slug).toBe("test-slug");
+  });
+
+  it("throws ArticleNotFoundError when slug is missing", async () => {
+    const repo = new MockArticlesRepository();
+    const useCase = getArticleBySlugUseCase(repo);
+    await expect(useCase({ slug: "does-not-exist" })).rejects.toThrow(ArticleNotFoundError);
+  });
+});