Add Contact Management Functionality

- Introduced a new contact management feature, including the ability to create, retrieve, search, update, and delete contact messages.
- Implemented the Contact entity, repository, service, and handler layers following Clean Architecture principles.
- Added API endpoints for contact operations in Swagger documentation, ensuring comprehensive API specifications.
- Enhanced error handling and validation for contact data, improving robustness and user experience.
- Updated the main application to initialize the contact repository and service, integrating them into the existing system.

internal/contact/errors.go

@@ -0,0 +1,18 @@
+package contact
+
+import "errors"
+
+// Custom errors for contact domain
+var (
+	ErrContactNotFound        = errors.New("contact not found")
+	ErrContactAlreadyResolved = errors.New("contact is already resolved")
+	ErrContactClosed          = errors.New("contact is closed and cannot be updated")
+	ErrInvalidStatus          = errors.New("invalid contact status")
+	ErrInvalidDateRange       = errors.New("invalid date range: date_from must be before date_to")
+	ErrUnauthorizedAccess     = errors.New("unauthorized access to contact")
+	ErrInvalidContactData     = errors.New("invalid contact data")
+	ErrEmailRequired          = errors.New("email is required")
+	ErrPhoneRequired          = errors.New("phone is required")
+	ErrMessageRequired        = errors.New("message is required")
+	ErrFullNameRequired       = errors.New("full name is required")
+)