refactor(marketing-pages): factory-style use cases + per-use-case controllers

- Use cases (get-page-by-slug, get-site-settings) → factory functions with I*UseCase aliases
- Controllers split: pages.controller.ts → 2 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: §6.3

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

packages/marketing-pages/src/di/symbols.ts

@@ -1,4 +1,10 @@
 export const MARKETING_PAGES_SYMBOLS = {
   IPagesRepository: Symbol.for("marketing-pages:IPagesRepository"),
   ISiteSettingsRepository: Symbol.for("marketing-pages:ISiteSettingsRepository"),
+  // Use cases
+  IGetPageBySlugUseCase: Symbol.for("marketing-pages:IGetPageBySlugUseCase"),
+  IGetSiteSettingsUseCase: Symbol.for("marketing-pages:IGetSiteSettingsUseCase"),
+  // Controllers
+  IGetPageBySlugController: Symbol.for("marketing-pages:IGetPageBySlugController"),
+  IGetSiteSettingsController: Symbol.for("marketing-pages:IGetSiteSettingsController"),
 } as const;