refactor(channelserver): extract GuildRepository for guild table access

Per anti-patterns.md item #9, guild-related SQL was scattered across
~15 handler files with no repository abstraction. Following the same
pattern established by CharacterRepository, this centralizes all
guilds, guild_characters, and guild_applications table access into a
single GuildRepository (~30 methods).

guild_model.go and handlers_guild_member.go are trimmed to types and
pure business logic only. All handler files (guild_*, festa, mail,
house, mercenary, rengoku) now call s.server.guildRepo methods
instead of direct DB queries or methods on domain objects.

server/channelserver/handlers_mail.go

@@ -339,13 +339,13 @@ func handleMsgMhfSendMail(s *Session, p mhfpacket.MHFPacket) {
 	`
 
 	if pkt.RecipientID == 0 { // Guild mail
-		g, err := GetGuildInfoByCharacterId(s, s.charID)
+		g, err := s.server.guildRepo.GetByCharID(s.charID)
 		if err != nil {
 			s.logger.Error("Failed to get guild info for mail")
 			doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
 			return
 		}
-		gm, err := GetGuildMembers(s, g.ID, false)
+		gm, err := s.server.guildRepo.GetMembers(g.ID, false)
 		if err != nil {
 			s.logger.Error("Failed to get guild members for mail")
 			doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))