Refactor customer domain structure and implement core entities, services, and handlers

- Introduced a flat structure for the customer domain, consolidating entities, aggregates, repositories, services, forms, and handlers into a single package.
- Added core entities: Customer and CustomerAggregate.
- Implemented request/response forms for customer authentication (Register, Login, Refresh Token, Change Password).
- Created CustomerService to handle business logic and data access through the Repository interface.
- Established CustomerHandler for HTTP request handling related to customer authentication.
- Removed previous domain structure artifacts to streamline the codebase.

internal/customer/aggregate.go

@@ -0,0 +1,46 @@
+package customer
+
+import (
+	"github.com/google/uuid"
+)
+
+type CustomerAggregate struct {
+	ID          uuid.UUID `json:"_id"`
+	FirstName   string    `json:"first_name"`
+	LastName    string    `json:"last_name"`
+	Mobile      string    `json:"mobile"`
+	Email       string    `json:"email"`
+	IsActive    bool      `json:"is_active"`
+	IsVerified  bool      `json:"is_verified"`
+	LastLoginAt *int64    `json:"last_login_at,omitempty"` // Unix timestamp
+	CreatedAt   int64     `json:"created_at"`              // Unix timestamp
+	UpdatedAt   int64     `json:"updated_at"`              // Unix timestamp
+}
+
+func NewCustomerAggregate(c Customer) CustomerAggregate {
+	customer := CustomerAggregate{
+		ID:         c.ID,
+		FirstName:  c.FirstName,
+		LastName:   c.LastName,
+		Mobile:     c.Mobile,
+		Email:      c.Email,
+		IsActive:   c.IsActive,
+		IsVerified: c.IsVerified,
+		CreatedAt:  c.CreatedAt,
+		UpdatedAt:  c.UpdatedAt,
+	}
+
+	if c.LastLoginAt != nil {
+		customer.LastLoginAt = c.LastLoginAt
+	}
+
+	return customer
+}
+
+// AuthResponse defines authentication response (kept for backward compatibility)
+type AuthorizationResponse struct {
+	Customer     CustomerAggregate `json:"customer"`
+	AccessToken  string            `json:"access_token"`
+	RefreshToken string            `json:"refresh_token"`
+	ExpiresIn    int64             `json:"expires_in"`
+}