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/internal/media/media.go

@@ -0,0 +1,46 @@
+package media
+
+import (
+	"errors"
+	"sync"
+
+	"context"
+	"github.com/QuantumNous/new-api/relaykit/types"
+)
+
+type MediaResolver struct {
+	GetBase64Data        func(c context.Context, source types.FileSource, reason ...string) (string, string, error)
+	DecodeBase64FileData func(base64String string) (string, string, error)
+}
+
+var (
+	mediaResolverMu sync.RWMutex
+	mediaResolver   MediaResolver
+)
+
+func SetMediaResolver(resolver MediaResolver) {
+	mediaResolverMu.Lock()
+	defer mediaResolverMu.Unlock()
+
+	mediaResolver = resolver
+}
+
+func ResolveBase64Data(c context.Context, source types.FileSource, reason ...string) (string, string, error) {
+	mediaResolverMu.RLock()
+	resolver := mediaResolver.GetBase64Data
+	mediaResolverMu.RUnlock()
+	if resolver == nil {
+		return "", "", errors.New("relayconvert media resolver is not configured")
+	}
+	return resolver(c, source, reason...)
+}
+
+func DecodeBase64FileData(base64String string) (string, string, error) {
+	mediaResolverMu.RLock()
+	resolver := mediaResolver.DecodeBase64FileData
+	mediaResolverMu.RUnlock()
+	if resolver == nil {
+		return "", "", errors.New("relayconvert media resolver is not configured")
+	}
+	return resolver(base64String)
+}