feat(account): make email optional for anonymous accounts

Changes:
- Modified CreateAccountRequest to make email optional (omitempty)
- Changed Account.Email from string to *string pointer type
- Updated PostgreSQL repository to handle nullable email with sql.NullString
- Username remains required and auto-generated by identity-service

This supports anonymous account creation without requiring email registration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/mpc-system/services/account/adapters/input/http/account_handler.go

@@ -108,7 +108,7 @@ func (h *AccountHTTPHandler) RegisterRoutes(router *gin.RouterGroup) {
 // CreateAccountRequest represents the request for creating an account
 type CreateAccountRequest struct {
 	Username        string        `json:"username" binding:"required"`
-	Email           string        `json:"email" binding:"required,email"`
+	Email           string        `json:"email" binding:"omitempty,email"`
 	Phone           *string       `json:"phone"`
 	PublicKey       string        `json:"publicKey" binding:"required"`
 	KeygenSessionID string        `json:"keygenSessionId" binding:"required"`

backend/mpc-system/services/account/adapters/output/postgres/account_repo.go

@@ -215,7 +215,7 @@ func (r *AccountPostgresRepo) scanAccount(row *sql.Row) (*entities.Account, erro
 	var (
 		id              uuid.UUID
 		username        string
-		email           string
+		email           sql.NullString
 		phone           sql.NullString
 		publicKey       []byte
 		keygenSessionID uuid.UUID
@@ -249,7 +249,9 @@ func (r *AccountPostgresRepo) scanAccount(row *sql.Row) (*entities.Account, erro
 
 	account.ID = value_objects.AccountIDFromUUID(id)
 	account.Username = username
-	account.Email = email
+	if email.Valid {
+		account.Email = &email.String
+	}
 	if phone.Valid {
 		account.Phone = &phone.String
 	}
@@ -267,7 +269,7 @@ func (r *AccountPostgresRepo) scanAccountFromRows(rows *sql.Rows) (*entities.Acc
 	var (
 		id              uuid.UUID
 		username        string
-		email           string
+		email           sql.NullString
 		phone           sql.NullString
 		publicKey       []byte
 		keygenSessionID uuid.UUID
@@ -298,7 +300,9 @@ func (r *AccountPostgresRepo) scanAccountFromRows(rows *sql.Rows) (*entities.Acc
 
 	account.ID = value_objects.AccountIDFromUUID(id)
 	account.Username = username
-	account.Email = email
+	if email.Valid {
+		account.Email = &email.String
+	}
 	if phone.Valid {
 		account.Phone = &phone.String
 	}

backend/mpc-system/services/account/domain/entities/account.go

@@ -10,8 +10,8 @@ import (
 // Account represents a user account with MPC-based authentication
 type Account struct {
 	ID              value_objects.AccountID
-	Username        string
-	Email           string
+	Username        string  // Required: auto-generated by identity-service
+	Email           *string // Optional: for anonymous accounts
 	Phone           *string
 	PublicKey       []byte // MPC group public key
 	KeygenSessionID uuid.UUID
@@ -33,10 +33,16 @@ func NewAccount(
 	thresholdT int,
 ) *Account {
 	now := time.Now().UTC()
+	var emailPtr *string
+
+	if email != "" {
+		emailPtr = &email
+	}
+
 	return &Account{
 		ID:              value_objects.NewAccountID(),
 		Username:        username,
-		Email:           email,
+		Email:           emailPtr,
 		PublicKey:       publicKey,
 		KeygenSessionID: keygenSessionID,
 		ThresholdN:      thresholdN,
@@ -119,9 +125,7 @@ func (a *Account) Validate() error {
 	if a.Username == "" {
 		return ErrInvalidUsername
 	}
-	if a.Email == "" {
-		return ErrInvalidEmail
-	}
+	// Email is optional, but if provided must be valid (checked by binding)
 	if len(a.PublicKey) == 0 {
 		return ErrInvalidPublicKey
 	}