refactor(guild): introduce service layer for guild member operations

Extract business logic from handleMsgMhfOperateGuildMember into
GuildService.OperateMember, establishing the handler→service→repo
layering pattern. The handler is now ~20 lines of protocol glue
(type-assert, map action, call service, send ACK, notify).

GuildService owns authorization checks, repo coordination, mail
composition, and best-effort mail delivery. It accepts plain Go
types (no mhfpacket or Session imports), making it fully testable
with mock repos. Cross-channel notification stays in the handler
since it requires Session.

Adds 7 table-driven service-level tests covering accept/reject/kick,
authorization, repo errors, mail errors, and unknown actions.

server/channelserver/sys_channel_server.go

@@ -71,6 +71,7 @@ type Server struct {
 	miscRepo        MiscRepo
 	scenarioRepo    ScenarioRepo
 	mercenaryRepo   MercenaryRepo
+	guildService    *GuildService
 	erupeConfig     *cfg.Config
 	acceptConns     chan net.Conn
 	deleteConns     chan net.Conn
@@ -153,6 +154,8 @@ func NewServer(config *Config) *Server {
 	s.scenarioRepo = NewScenarioRepository(config.DB)
 	s.mercenaryRepo = NewMercenaryRepository(config.DB)
 
+	s.guildService = NewGuildService(s.guildRepo, s.mailRepo, s.charRepo, s.logger)
+
 	// Mezeporta
 	s.stages.Store("sl1Ns200p0a0u0", NewStage("sl1Ns200p0a0u0"))