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/server/api_contract_test.go

@@ -178,6 +178,10 @@ func TestAPIContracts(t *testing.T) {
 						"image_price_1k": null,
 						"image_price_2k": null,
 						"image_price_4k": null,
+						"sora_image_price_360": null,
+						"sora_image_price_540": null,
+						"sora_video_price_per_request": null,
+						"sora_video_price_per_request_hd": null,
 						"claude_code_only": false,
 						"fallback_group_id": null,
 						"created_at": "2025-01-02T03:04:05Z",
@@ -394,6 +398,7 @@ func TestAPIContracts(t *testing.T) {
 							"first_token_ms": 50,
 							"image_count": 0,
 							"image_size": null,
+							"media_type": null,
 							"created_at": "2025-01-02T03:04:05Z",
 							"user_agent": null
 						}
@@ -887,6 +892,10 @@ func (s *stubAccountRepo) GetByCRSAccountID(ctx context.Context, crsAccountID st
 	return nil, errors.New("not implemented")
 }
 
+func (s *stubAccountRepo) FindByExtraField(ctx context.Context, key string, value any) ([]service.Account, error) {
+	return nil, errors.New("not implemented")
+}
+
 func (s *stubAccountRepo) Update(ctx context.Context, account *service.Account) error {
 	return errors.New("not implemented")
 }