refactor(marketing-pages): unify use-case I/O schemas + presenter + feature error map

Per Plan 9 (spec R1-R28):
- Use cases: input + output schemas (getPageBySlug, getSiteSettings).
  Site-settings input is z.object({}).strict() per R5 (uniform input).
- Controllers: unknown input + identity presenter; void output not
  applicable (both use cases return data).
- New integrations/api/procedures.ts with marketingPagesProcedure
  ([InputParseError → BAD_REQUEST], [PageNotFoundError → NOT_FOUND]).
- Router uses marketingPagesProcedure + .input(xInputSchema).
- src/index.ts: remove pageBySlugQuery/siteSettingsQuery; export
  schemas + types + IUseCase/IController aliases.
- src/ui/index.ts (NEW); package.json adds ./ui subpath.
- R25 output-validation tests + R26 router error-mapping test.

Refactor log: §1, §2, §3.1, §3.2, §3.3, §5.1, §5.2, §6.1, §6.2
Spec: R1–R6, R8–R15, R18–R20, R22–R26

packages/marketing-pages/package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "exports": {
     ".": "./src/index.ts",
+    "./ui": "./src/ui/index.ts",
     "./cms": "./src/integrations/cms/index.ts",
     "./api": "./src/integrations/api/router.ts",
     "./di/bind-production": "./src/di/bind-production.ts"