Enhance Dockerfile to build and include a new HTTP API service (mtproxy_checkerd) alongside the existing CLI tool (mtproxy_checker). Update README and documentation to reflect the new service and its usage, including environment variables and Docker run instructions.

internal/checkresult/checkresult.go

@@ -0,0 +1,22 @@
+package checkresult
+
+import (
+	"context"
+	"errors"
+
+	"mtproxy_checker/internal/checker"
+)
+
+// Classify maps a checker error to CLI-style exit codes: 0 OK, 1 generic, 3 proxy closed, 4 timeout.
+func Classify(err error) (exitCode int, message string) {
+	if err == nil {
+		return 0, ""
+	}
+	if errors.Is(err, checker.ErrProxyClosed) {
+		return 3, err.Error()
+	}
+	if errors.Is(err, context.DeadlineExceeded) {
+		return 4, "timeout"
+	}
+	return 1, err.Error()
+}