Fix: Normalize userId to integer in POST/PUT requestsFix: Normalize userId to integer in POST/PUT requests

[Tania2607]

When using the /posts endpoint:

• GET /posts/1 returns userId as an integer:
  { "userId": 1, ... }

• POST /posts with userId=1 returns it as a string:
  { "userId": "1", ... }

This causes issues in strictly typed languages like Swift or TypeScript.

Suggested fix: Normalize userId as an integer across all responses.

Fixes: #217