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.

config/config.go

@@ -337,7 +337,7 @@ func getOutboundIP4() net.IP {
 	if err != nil {
 		log.Fatal(err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	localAddr := conn.LocalAddr().(*net.UDPAddr)