feat(backend): Enable input validation and response envelope 🛡️ - Ad…

[datagerl]

Summary 🎉

Aligns the active backend with its documented conventions for input validation and response shape!

Changes Made 🚀

1. Enabled Global Request Validation (main.ts:10-14):

   • Added ValidationPipe with whitelist: true, forbidNonWhitelisted: true, and transform: true
   • Now invalid payloads are caught automatically!

2. Created Response Transform Interceptor (src/common/interceptors/transform.interceptor.ts):

   • Wraps all successful responses in ApiResponseDto envelope
   • Maintains flexibility for responses already using ApiResponseDto

3. Created Http Exception Filter (src/common/filters/http-exception.filter.ts):

   • Catches all HttpExceptions (including validation errors)
   • Returns errors in predictable ApiResponseDto format with success: false

Acceptance Criteria Check ✅

• Global request validation is enabled.
• Active routes return a consistent envelope where intended.
• Invalid payloads fail with predictable formatting.

closes #197