docs: add comprehensive K6 load testing guide with API structure

- Document all API endpoints (auth, listings, payments, search)
- Include DTOs and request/response body shapes
- Document authentication methods and rate limits
- Provide database and environment configuration
- Include existing test setup (Playwright, Vitest)
- Detail CI/CD pipeline structure
- Recommend K6 endpoints and test patterns
- Provide file location references for quick lookup

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

apps/web/i18n/config.ts

@@ -0,0 +1,3 @@
+export const locales = ['vi', 'en'] as const;
+export type Locale = (typeof locales)[number];
+export const defaultLocale: Locale = 'vi';