fix(channelserver): resolve all golangci-lint issues and add handler tests

Fix errcheck violations across 11 repo files by wrapping deferred
rows.Close() and tx.Rollback() calls to discard the error return.
Fix unchecked Scan/Exec calls in guild store tests. Fix staticcheck
SA9003 empty branch in test helpers.

Add 6 mock-based unit tests for GetCharacterSaveData covering nil
savedata, sql.ErrNoRows, DB errors, compressed round-trip,
new-character skip, and config mode/pointer propagation.

server/channelserver/repo_tower.go

@@ -124,7 +124,7 @@ func (r *TowerRepository) GetTenrouiraiMissionScores(guildID uint32, missionInde
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 	var scores []TenrouiraiCharScore
 	for rows.Next() {
 		var cs TenrouiraiCharScore