refactor(mail): extract mail logic into MailService

Introduce MailService as a convenience layer between handlers/services
and MailRepo. Provides Send, SendSystem, SendGuildInvite, and
BroadcastToGuild methods that encapsulate the boolean flag combinations.

GuildService now depends on MailService instead of MailRepo directly,
simplifying its mail-sending calls from verbose SendMail(..., false, true)
to clean SendSystem(recipientID, subject, body).

Guild mail broadcast logic moved from handleMsgMhfSendMail into
MailService.BroadcastToGuild.

server/channelserver/svc_mail_test.go

@@ -0,0 +1,162 @@
+package channelserver
+
+import (
+	"errors"
+	"testing"
+
+	"go.uber.org/zap"
+)
+
+func TestMailService_Send(t *testing.T) {
+	mock := &mockMailRepo{}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mock, nil, logger)
+
+	err := svc.Send(1, 42, "Hello", "World", 500, 3)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if len(mock.sentMails) != 1 {
+		t.Fatalf("Expected 1 mail, got %d", len(mock.sentMails))
+	}
+	m := mock.sentMails[0]
+	if m.senderID != 1 {
+		t.Errorf("SenderID = %d, want 1", m.senderID)
+	}
+	if m.recipientID != 42 {
+		t.Errorf("RecipientID = %d, want 42", m.recipientID)
+	}
+	if m.subject != "Hello" {
+		t.Errorf("Subject = %q, want %q", m.subject, "Hello")
+	}
+	if m.itemID != 500 {
+		t.Errorf("ItemID = %d, want 500", m.itemID)
+	}
+	if m.itemAmount != 3 {
+		t.Errorf("Quantity = %d, want 3", m.itemAmount)
+	}
+	if m.isGuildInvite || m.isSystemMessage {
+		t.Error("Should not be guild invite or system message")
+	}
+}
+
+func TestMailService_Send_Error(t *testing.T) {
+	mock := &mockMailRepo{sendErr: errors.New("db fail")}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mock, nil, logger)
+
+	err := svc.Send(1, 42, "Hello", "World", 0, 0)
+	if err == nil {
+		t.Fatal("Expected error, got nil")
+	}
+}
+
+func TestMailService_SendSystem(t *testing.T) {
+	mock := &mockMailRepo{}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mock, nil, logger)
+
+	err := svc.SendSystem(42, "System Alert", "Something happened")
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if len(mock.sentMails) != 1 {
+		t.Fatalf("Expected 1 mail, got %d", len(mock.sentMails))
+	}
+	m := mock.sentMails[0]
+	if m.senderID != 0 {
+		t.Errorf("SenderID = %d, want 0 (system)", m.senderID)
+	}
+	if m.recipientID != 42 {
+		t.Errorf("RecipientID = %d, want 42", m.recipientID)
+	}
+	if !m.isSystemMessage {
+		t.Error("Should be system message")
+	}
+	if m.isGuildInvite {
+		t.Error("Should not be guild invite")
+	}
+}
+
+func TestMailService_SendGuildInvite(t *testing.T) {
+	mock := &mockMailRepo{}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mock, nil, logger)
+
+	err := svc.SendGuildInvite(1, 42, "Invite", "Join us")
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if len(mock.sentMails) != 1 {
+		t.Fatalf("Expected 1 mail, got %d", len(mock.sentMails))
+	}
+	m := mock.sentMails[0]
+	if !m.isGuildInvite {
+		t.Error("Should be guild invite")
+	}
+	if m.isSystemMessage {
+		t.Error("Should not be system message")
+	}
+}
+
+func TestMailService_BroadcastToGuild(t *testing.T) {
+	mailMock := &mockMailRepo{}
+	guildMock := &mockGuildRepoForMail{
+		members: []*GuildMember{
+			{CharID: 100},
+			{CharID: 200},
+			{CharID: 300},
+		},
+	}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mailMock, guildMock, logger)
+
+	err := svc.BroadcastToGuild(1, 10, "News", "Update")
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if len(mailMock.sentMails) != 3 {
+		t.Fatalf("Expected 3 mails, got %d", len(mailMock.sentMails))
+	}
+	recipients := map[uint32]bool{}
+	for _, m := range mailMock.sentMails {
+		recipients[m.recipientID] = true
+		if m.senderID != 1 {
+			t.Errorf("SenderID = %d, want 1", m.senderID)
+		}
+	}
+	if !recipients[100] || !recipients[200] || !recipients[300] {
+		t.Errorf("Expected recipients 100, 200, 300, got %v", recipients)
+	}
+}
+
+func TestMailService_BroadcastToGuild_GetMembersError(t *testing.T) {
+	mailMock := &mockMailRepo{}
+	guildMock := &mockGuildRepoForMail{getMembersErr: errors.New("db fail")}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mailMock, guildMock, logger)
+
+	err := svc.BroadcastToGuild(1, 10, "News", "Update")
+	if err == nil {
+		t.Fatal("Expected error, got nil")
+	}
+	if len(mailMock.sentMails) != 0 {
+		t.Errorf("No mails should be sent on error, got %d", len(mailMock.sentMails))
+	}
+}
+
+func TestMailService_BroadcastToGuild_SendError(t *testing.T) {
+	mailMock := &mockMailRepo{sendErr: errors.New("db fail")}
+	guildMock := &mockGuildRepoForMail{
+		members: []*GuildMember{
+			{CharID: 100},
+		},
+	}
+	logger, _ := zap.NewDevelopment()
+	svc := NewMailService(mailMock, guildMock, logger)
+
+	err := svc.BroadcastToGuild(1, 10, "News", "Update")
+	if err == nil {
+		t.Fatal("Expected error, got nil")
+	}
+}