feat: use six-digit numeric member invite codes

internal/accounts/service_test.go

@@ -48,7 +48,8 @@ func TestRegisterValidation(t *testing.T) {
 		{name: "password", input: RegisterInput{Username: "alice", Phone: "13800138000", Password: "123", PrivacyAccepted: true}, want: ErrPasswordInvalid},
 		{name: "referral required", input: RegisterInput{Username: "alice", Phone: "13800138000", Password: "secret1", PrivacyAccepted: true}, want: ErrReferralInvalid},
 		{name: "referral", input: RegisterInput{Username: "alice", Phone: "13800138000", Password: "secret1", ReferralCode: "bad!", PrivacyAccepted: true}, want: ErrReferralInvalid},
-		{name: "privacy", input: RegisterInput{Username: "alice", Phone: "13800138000", Password: "secret1", ReferralCode: "ABC123"}, want: ErrPrivacyNotAccepted},
+		{name: "referral length", input: RegisterInput{Username: "alice", Phone: "13800138000", Password: "secret1", ReferralCode: "12345", PrivacyAccepted: true}, want: ErrReferralInvalid},
+		{name: "privacy", input: RegisterInput{Username: "alice", Phone: "13800138000", Password: "secret1", ReferralCode: "123456"}, want: ErrPrivacyNotAccepted},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
@@ -60,6 +61,15 @@ func TestRegisterValidation(t *testing.T) {
 	}
 }
 
+func TestNewInviteCodeIsSixDigits(t *testing.T) {
+	for i := 0; i < 100; i++ {
+		code, err := newInviteCode()
+		if err != nil || !referralPattern.MatchString(code) {
+			t.Fatalf("newInviteCode() = %q, %v", code, err)
+		}
+	}
+}
+
 func TestValidMemberNameSupportsChinese(t *testing.T) {
 	for _, name := range []string{"杨情", "欧阳娜娜", "Alicia", "阿依古丽·买买提"} {
 		if !validMemberName(name) {