Resolve "feat(backend): automatically update attended patients count on registration"

Feature Request Template

Title (semantic)

Format: feat(scope): short description Example: feat(auth): add otp expiration check

Description

This merge request implements a mechanism to automatically update the attended_patients count in the MedicalCamp model whenever a patient is successfully registered via the POST /api/v1/patients/{book_no}/register endpoint. This ensures that the camp statistics allow for real-time tracking of attendance without requiring expensive aggregations on every read.

Checklist

• The feature has been fully implemented.
• Tests for the new feature are included and passing.
• User documentation/guides have been updated (if applicable).
• Impact on existing functionality has been considered.

Related Issue(s)

Closes #33 (closed)