Refactor project layout and add remote write control

[Suhaibinator]

Summary

• move main to cmd/smq
• move server code under internal/server
• allow disabling remote write calls with ALLOW_REMOTE_WRITES and --allow-remote-writes
• document new configuration
• enable remote writes in tests

[Copilot]

Pull Request Overview

This PR refactors the project layout while adding support for controlling remote write operations via a new configuration flag and environment variable. Key changes include moving server code under internal/server, introducing the ALLOW_REMOTE_WRITES configuration (and corresponding command-line flag), and updating tests and documentation to reflect this new feature.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/server/server_test.go	Updated import path and set allowRemoteWrites to true in tests.
internal/server/server.go	Added allowRemoteWrites flag checks to restrict remote write operations.
errors/errors.go	Added a new error constant for disabled remote writes.
config/configuration.go	Introduced configuration constant and variable for remote writes.
cmd/smq/main.go	Included a new flag/option to control remote write operations.
README.md	Documented the new remote writes configuration option.

[Copilot]

[nitpick] Consider abstracting the remote write permission check into a helper function to avoid repetitive code across multiple endpoints.

Suggested change

	func (s *Server) Produce(ctx context.Context, pr *pb.ProduceRequest) (*pb.ProduceResponse, error) {
	if !s.allowRemoteWrites {
	return nil, smqerrors.ErrRemoteWritesDisabled
	}
	func (s *Server) checkRemoteWritePermission() error {
	if !s.allowRemoteWrites {
	return smqerrors.ErrRemoteWritesDisabled
	}
	return nil
	}
	func (s *Server) Produce(ctx context.Context, pr *pb.ProduceRequest) (*pb.ProduceResponse, error) {
	if err := s.checkRemoteWritePermission(); err != nil {
	return nil, err
	}