## Issue #11: Implement Data Validation

### Description

Implement comprehensive data validation for all API inputs to ensure data integrity and prevent security vulnerabilities.

### Acceptance Criteria

- [ ] Create validation middleware
- [ ] Define validation rules for all endpoints
- [ ] Add error handling for validation failures
- [ ] Integrate validation with existing routes
- [ ] Document validation rules

### Tasks

- [ ] Install and configure Joi or express-validator
- [ ] Create validation middleware
- [ ] Define validation schemas for API endpoints
- [ ] Apply validation to all routes
- [ ] Add comprehensive error messages for validation failures

### Notes

This issue addresses the need for robust data validation to ensure data integrity and prevent security vulnerabilities. The implementation will use a validation library to define rules for all API inputs, with proper error handling and clear error messages for validation failures.