refactor(channelserver): extract RengokuRepository and MailRepository

Move all direct DB access from handlers_rengoku.go (11 calls) and
handlers_mail.go (10 calls) into dedicated repository types, continuing
the established extraction pattern.

RengokuRepository provides UpsertScore and GetRanking, replacing a
3-call check/insert/update sequence and an 8-case switch of nearly
identical queries respectively.

MailRepository provides SendMail, SendMailTx, GetListForCharacter,
GetByID, MarkRead, MarkDeleted, SetLocked, and MarkItemReceived.
The old Mail.Send(), Mail.MarkRead(), GetMailListForCharacter, and
GetMailByID free functions are removed. Guild handlers that sent mail
via Mail.Send(s, ...) now call mailRepo directly.

server/channelserver/sys_channel_server.go

@@ -52,6 +52,8 @@ type Server struct {
 	houseRepo      *HouseRepository
 	festaRepo      *FestaRepository
 	towerRepo      *TowerRepository
+	rengokuRepo    *RengokuRepository
+	mailRepo       *MailRepository
 	erupeConfig    *_config.Config
 	acceptConns    chan net.Conn
 	deleteConns    chan net.Conn
@@ -129,6 +131,8 @@ func NewServer(config *Config) *Server {
 	s.houseRepo = NewHouseRepository(config.DB)
 	s.festaRepo = NewFestaRepository(config.DB)
 	s.towerRepo = NewTowerRepository(config.DB)
+	s.rengokuRepo = NewRengokuRepository(config.DB)
+	s.mailRepo = NewMailRepository(config.DB)
 
 	// Mezeporta
 	s.stages["sl1Ns200p0a0u0"] = NewStage("sl1Ns200p0a0u0")