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/signserver/sign_server_test.go

@@ -434,7 +434,7 @@ func TestServerHandleConnection(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Dial() error: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	nullInit := make([]byte, 8)
 	_, err = conn.Write(nullInit)
@@ -472,7 +472,7 @@ func TestServerHandleConnectionWithShortInit(t *testing.T) {
 	}
 
 	_, _ = conn.Write([]byte{0, 0, 0, 0})
-	conn.Close()
+	_ = conn.Close()
 
 	time.Sleep(50 * time.Millisecond)
 }
@@ -502,7 +502,7 @@ func TestServerHandleConnectionImmediateClose(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Dial() error: %v", err)
 	}
-	conn.Close()
+	_ = conn.Close()
 
 	time.Sleep(50 * time.Millisecond)
 }
@@ -544,7 +544,7 @@ func TestServerMultipleConnections(t *testing.T) {
 	time.Sleep(50 * time.Millisecond)
 
 	for _, conn := range conns {
-		conn.Close()
+		_ = conn.Close()
 	}
 }