refactor(channelserver): extract UserRepository for users table access

Centralizes all 31 direct users-table SQL queries from 11 handler
files into a single UserRepository, following the same pattern as
CharacterRepository and GuildRepository. The only excluded query is
the sign_sessions JOIN in handleMsgSysLogin which spans multiple
tables.

server/channelserver/sys_channel_server.go

@@ -47,6 +47,7 @@ type Server struct {
 	db             *sqlx.DB
 	charRepo       *CharacterRepository
 	guildRepo      *GuildRepository
+	userRepo       *UserRepository
 	erupeConfig    *_config.Config
 	acceptConns    chan net.Conn
 	deleteConns    chan net.Conn
@@ -119,6 +120,7 @@ func NewServer(config *Config) *Server {
 
 	s.charRepo = NewCharacterRepository(config.DB)
 	s.guildRepo = NewGuildRepository(config.DB)
+	s.userRepo = NewUserRepository(config.DB)
 
 	// Mezeporta
 	s.stages["sl1Ns200p0a0u0"] = NewStage("sl1Ns200p0a0u0")