feat: 算力引擎 Go 后端核心（new-api）

- 统一 OpenAI 兼容 /v1 中继 + 渠道/额度/令牌/流水
- 各领域包：relay 模型网关、model 数据层、controller 管理 API

relaykit/relayconvert/kitutil/json.go

@@ -0,0 +1,86 @@
+// Package kitutil holds the dependency-free helpers shared by the conversion
+// kit packages (dto, types, relayconvert). It moved out of the host's common
+// package as part of the relaykit extraction; common re-exports these for
+// host code.
+package kitutil
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"unsafe"
+)
+
+func Unmarshal(data []byte, v any) error {
+	return json.Unmarshal(data, v)
+}
+
+func UnmarshalJsonStr(data string, v any) error {
+	return json.Unmarshal(StringToByteSlice(data), v)
+}
+
+func DecodeJson(reader io.Reader, v any) error {
+	return json.NewDecoder(reader).Decode(v)
+}
+
+func Marshal(v any) ([]byte, error) {
+	return json.Marshal(v)
+}
+
+func GetJsonType(data json.RawMessage) string {
+	trimmed := bytes.TrimSpace(data)
+	if len(trimmed) == 0 {
+		return "unknown"
+	}
+	firstChar := trimmed[0]
+	switch firstChar {
+	case '{':
+		return "object"
+	case '[':
+		return "array"
+	case '"':
+		return "string"
+	case 't', 'f':
+		return "boolean"
+	case 'n':
+		return "null"
+	default:
+		return "number"
+	}
+}
+
+// JsonRawMessageToString returns JSON strings as their decoded value and other JSON values as raw text.
+func JsonRawMessageToString(data json.RawMessage) string {
+	trimmed := bytes.TrimSpace(data)
+	if len(trimmed) == 0 || bytes.Equal(trimmed, []byte("null")) {
+		return ""
+	}
+	if trimmed[0] != '"' {
+		return string(trimmed)
+	}
+	var value string
+	if err := Unmarshal(trimmed, &value); err != nil {
+		return string(trimmed)
+	}
+	return value
+}
+
+func StringToByteSlice(s string) []byte {
+	tmp1 := (*[2]uintptr)(unsafe.Pointer(&s))
+	tmp2 := [3]uintptr{tmp1[0], tmp1[1], tmp1[1]}
+	return *(*[]byte)(unsafe.Pointer(&tmp2))
+}
+
+func Any2Type[T any](data any) (T, error) {
+	var zero T
+	bytes, err := json.Marshal(data)
+	if err != nil {
+		return zero, err
+	}
+	var res T
+	err = json.Unmarshal(bytes, &res)
+	if err != nil {
+		return zero, err
+	}
+	return res, nil
+}