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_mercenary.go

@@ -388,7 +388,7 @@ type Airou struct {
 func getGuildAirouList(s *Session) []Airou {
 	var guildCats []Airou
 	bannedCats := make(map[uint32]int)
-	guild, err := GetGuildInfoByCharacterId(s, s.charID)
+	guild, err := s.server.guildRepo.GetByCharID(s.charID)
 	if err != nil {
 		return guildCats
 	}