fix: check all Close() return values for errcheck lint

Add explicit error discards (_ =) for Close() calls on network
connections, SQL rows, and file handles across 28 files. Also add
.golangci.yml with standard linter defaults to match CI configuration.

server/channelserver/handlers_mail.go

@@ -108,7 +108,7 @@ func GetMailListForCharacter(s *Session, charID uint32) ([]Mail, error) {
 		return nil, err
 	}
 
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	allMail := make([]Mail, 0)