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/api/api_server_test.go

@@ -124,7 +124,7 @@ func TestAPIServerStart(t *testing.T) {
 		// This might fail if the server didn't start properly or port is blocked
 		t.Logf("Failed to connect to server: %v", err)
 	} else {
-		defer resp.Body.Close()
+		defer func() { _ = resp.Body.Close() }()
 		if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusNotFound {
 			t.Logf("Unexpected status code: %d", resp.StatusCode)
 		}