Commit go.sum and fix CI lint failures

[jeanmachuca]

What

Two issues blocking CI:

1. go.sum is missing from the repository
2. golangci-lint reports multiple errcheck violations:
   • internal/server/server.go: unchecked json.NewEncoder(w).Encode, json.Decoder.Decode
   • internal/model/model_test.go: unchecked os.MkdirAll, os.WriteFile (test code)
   • internal/server/server_test.go: unchecked os.MkdirAll, json.Encoder.Encode (test code)

Why

CI is completely broken. While some violations are in test code, they should still be fixed for consistency.

How

• Run 'go mod tidy' and commit go.sum
• Add explicit error checks for all json.Encode/Decode and os.MkdirAll/WriteFile calls
• Verify CI passes after fixes