feat(Sora): 直连生成并移除sora2api依赖

实现直连 Sora 客户端、媒体落地与清理策略\n更新网关与前端配置以支持 Sora 平台\n补齐单元测试与契约测试，新增 curl 测试脚本\n\n测试: go test ./... -tags=unit

backend/internal/service/sora_client_test.go

@@ -0,0 +1,54 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSoraDirectClient_DoRequestSuccess(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`{"ok":true}`))
+	}))
+	defer server.Close()
+
+	cfg := &config.Config{
+		Sora: config.SoraConfig{
+			Client: config.SoraClientConfig{BaseURL: server.URL},
+		},
+	}
+	client := NewSoraDirectClient(cfg, nil, nil)
+
+	body, _, err := client.doRequest(context.Background(), &Account{ID: 1}, http.MethodGet, server.URL, http.Header{}, nil, false)
+	require.NoError(t, err)
+	require.Contains(t, string(body), "ok")
+}
+
+func TestSoraDirectClient_BuildBaseHeaders(t *testing.T) {
+	cfg := &config.Config{
+		Sora: config.SoraConfig{
+			Client: config.SoraClientConfig{
+				Headers: map[string]string{
+					"X-Test":                "yes",
+					"Authorization":         "should-ignore",
+					"openai-sentinel-token": "skip",
+				},
+			},
+		},
+	}
+	client := NewSoraDirectClient(cfg, nil, nil)
+
+	headers := client.buildBaseHeaders("token-123", "UA")
+	require.Equal(t, "Bearer token-123", headers.Get("Authorization"))
+	require.Equal(t, "UA", headers.Get("User-Agent"))
+	require.Equal(t, "yes", headers.Get("X-Test"))
+	require.Empty(t, headers.Get("openai-sentinel-token"))
+}