feat: add advanced input validation

[allisson]

Integrate the jellydator/validation library for comprehensive input validation across DTOs and use case layers, with custom validation rules for password strength, email format, and whitespace handling.

Changes:

• Add jellydator/validation v1.2.0 dependency
• Create internal/validation package with custom validation rules:
  • PasswordStrength: configurable password complexity validation
  • Email: regex-based email format validation
  • NoWhitespace: prevent leading/trailing whitespace
  • NotBlank: ensure non-empty strings after trimming
  • WrapValidationError: helper to convert validation errors to domain errors
• Update RegisterUserRequest DTO validation with:
  • Name: required, not blank, 1-255 characters
  • Email: required, valid format, 5-255 characters
  • Password: 8-128 chars, uppercase, lowercase, number, special character
• Update use case input validation with same comprehensive rules
• Validation errors wrapped as domain ErrInvalidInput (422 status)
• Add comprehensive test suite for all validation rules (100% coverage)
• Update test fixtures to use valid passwords meeting new requirements
• Update README.md with:
  • Input validation feature in features list
  • Validation package in project structure
  • Password requirements and validation error examples in usage section
  • New "Input Validation" section with detailed documentation
  • jellydator/validation in dependencies and acknowledgments

Benefits:

• Declarative and type-safe validation rules
• Reusable custom validators across the application
• Consistent validation at DTO and use case layers
• User-friendly error messages for API clients
• Enhanced security with password strength requirements
• Better data quality with format and constraint validation