[Good First Issue] Add email validation to subscription schema

## Description

The SubscriptionSchema doesn't validate emails correctly.

## Tasks
- [ ] Add Pydantic validator for emails using EmailStr
- [ ] Write unit test for validation failure
- [ ] Verify it returns 422 error with clear message

## Difficulty: ⭐⭐ Easy
## Time: 45 min

File: apps/api/app/schemas/subscription.py