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/symbols.ts

@@ -1,3 +1,11 @@
 export const BLOG_SYMBOLS = {
   IArticlesRepository: Symbol.for("blog:IArticlesRepository"),
+  // Use cases
+  IGetArticlesUseCase: Symbol.for("blog:IGetArticlesUseCase"),
+  ICreateArticleUseCase: Symbol.for("blog:ICreateArticleUseCase"),
+  IGetArticleBySlugUseCase: Symbol.for("blog:IGetArticleBySlugUseCase"),
+  // Controllers
+  IGetArticlesController: Symbol.for("blog:IGetArticlesController"),
+  ICreateArticleController: Symbol.for("blog:ICreateArticleController"),
+  IGetArticleBySlugController: Symbol.for("blog:IGetArticleBySlugController"),
 } as const;