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/di/container.test.ts

@@ -22,15 +22,18 @@ describe("blogContainer", () => {
     expect(repo).toBeInstanceOf(MockArticlesRepository);
   });
 
-  it("supports rebinding to a custom repo", () => {
-    blogContainer.unbind(BLOG_SYMBOLS.IArticlesRepository);
-    const custom = new MockArticlesRepository();
-    blogContainer
-      .bind<IArticlesRepository>(BLOG_SYMBOLS.IArticlesRepository)
-      .toConstantValue(custom);
-    const resolved = blogContainer.get<IArticlesRepository>(
-      BLOG_SYMBOLS.IArticlesRepository,
-    );
-    expect(resolved).toBe(custom);
+  it("resolves IGetArticlesController from the container", () => {
+    const ctrl = blogContainer.get(BLOG_SYMBOLS.IGetArticlesController);
+    expect(typeof ctrl).toBe("function");
+  });
+
+  it("resolves ICreateArticleController from the container", () => {
+    const ctrl = blogContainer.get(BLOG_SYMBOLS.ICreateArticleController);
+    expect(typeof ctrl).toBe("function");
+  });
+
+  it("resolves IGetArticleBySlugController from the container", () => {
+    const ctrl = blogContainer.get(BLOG_SYMBOLS.IGetArticleBySlugController);
+    expect(typeof ctrl).toBe("function");
   });
 });