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/entranceserver/entrance_server.go

@@ -65,7 +65,7 @@ func (s *Server) Shutdown() {
 	s.Unlock()
 
 	// This will cause the acceptor goroutine to error and exit gracefully.
-	s.listener.Close()
+	_ = s.listener.Close()
 }
 
 // acceptClients handles accepting new clients in a loop.
@@ -91,7 +91,7 @@ func (s *Server) acceptClients() {
 }
 
 func (s *Server) handleEntranceServerConnection(conn net.Conn) {
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 	// Client initalizes the connection with a one-time buffer of 8 NULL bytes.
 	nullInit := make([]byte, 8)
 	n, err := io.ReadFull(conn, nullInit)

server/entranceserver/entrance_server_test.go

@@ -445,7 +445,7 @@ func TestServerHandleConnectionImmediateClose(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Dial() error: %v", err)
 	}
-	conn.Close()
+	_ = conn.Close()
 
 	time.Sleep(50 * time.Millisecond)
 }
@@ -477,7 +477,7 @@ func TestServerHandleConnectionShortInit(t *testing.T) {
 		t.Fatalf("Dial() error: %v", err)
 	}
 	_, _ = conn.Write([]byte{0, 0, 0, 0})
-	conn.Close()
+	_ = conn.Close()
 
 	time.Sleep(50 * time.Millisecond)
 }
@@ -517,6 +517,6 @@ func TestServerMultipleConnections(t *testing.T) {
 	time.Sleep(50 * time.Millisecond)
 
 	for _, conn := range conns {
-		conn.Close()
+		_ = conn.Close()
 	}
 }