refactor: 统一任务框架 Phase 3 - 用 TaskService/FailoverEngine 替代 FallbackOrchestrator

核心重构：
- 移除 FallbackOrchestrator，用 TaskService + FailoverEngine 替代
- TaskService 作为统一入口，支持 SYNC/ASYNC 两种任务模式
- FailoverEngine 实现候选遍历、重试、故障转移逻辑
- 新增 AttemptFunc/AttemptResult 协议，统一尝试结果表示

功能改进：
- 流式响应首字节探测（30s 超时，空流触发故障转移）
- 流式取消归因优化（区分客户端断连 vs 服务端中断）
- 新增 OpenAI Sora 视频取消路由 POST /v1/videos/{task_id}/cancel
- OpenAI 流式请求自动添加 stream_options.include_usage

代码规范：
- 修复 loguru 日志格式（%s → {}）
- 新增 FORMAT_CONVERSION_ENABLED 环境变量说明

测试覆盖：
- test_failover_engine.py: FailoverEngine 单元测试
- test_task_service_async_execute.py: TaskService ASYNC 模式测试
- test_video_cancel_e2e.py: 视频取消端到端测试

tests/services/test_video_finalize.py

@@ -7,7 +7,7 @@ import pytest
 from src.config.settings import config
 from src.core.api_format.conversion.internal_video import VideoStatus
 from src.services.billing.formula_engine import BillingIncompleteError
-from src.services.task.application import TaskApplicationService
+from src.services.task.service import TaskService
 
 
 def _make_task(**overrides: Any) -> SimpleNamespace:
@@ -81,18 +81,18 @@ async def test_video_finalize_failed_records_cost_zero(monkeypatch: pytest.Monke
         lambda _self, **_kwargs: {"duration_seconds": 4},
     )
     monkeypatch.setattr(
-        "src.services.task.application.BillingRuleService.find_rule",
+        "src.services.billing.rule_service.BillingRuleService.find_rule",
         lambda *_args, **_kwargs: None,
     )
     # Mock update_settled_billing (used by finalize_video_task)
     update_settled = MagicMock(return_value=True)
     monkeypatch.setattr(
-        "src.services.task.application.UsageService.update_settled_billing",
+        "src.services.usage.service.UsageService.update_settled_billing",
         update_settled,
     )
 
-    app = TaskApplicationService(db)
-    await app.finalize_video_task(task)
+    svc = TaskService(db)
+    await svc.finalize_video_task(task)
 
     # billing_snapshot should be written back to task.request_metadata
     assert task.request_metadata["billing_snapshot"]["cost"] == 0.0
@@ -113,18 +113,18 @@ async def test_video_finalize_completed_no_rule(monkeypatch: pytest.MonkeyPatch)
         lambda _self, **_kwargs: {"duration_seconds": 4},
     )
     monkeypatch.setattr(
-        "src.services.task.application.BillingRuleService.find_rule",
+        "src.services.billing.rule_service.BillingRuleService.find_rule",
         lambda *_args, **_kwargs: None,
     )
     # Mock update_settled_billing (used by finalize_video_task)
     update_settled = MagicMock(return_value=True)
     monkeypatch.setattr(
-        "src.services.task.application.UsageService.update_settled_billing",
+        "src.services.usage.service.UsageService.update_settled_billing",
         update_settled,
     )
 
-    app = TaskApplicationService(db)
-    await app.finalize_video_task(task)
+    svc = TaskService(db)
+    await svc.finalize_video_task(task)
 
     assert task.request_metadata["billing_snapshot"]["status"] == "no_rule"
 
@@ -161,7 +161,7 @@ async def test_video_finalize_strict_mode_missing_required_marks_failed(
     # Mock update_settled_billing (used by finalize_video_task)
     update_settled = MagicMock(return_value=True)
     monkeypatch.setattr(
-        "src.services.task.application.UsageService.update_settled_billing",
+        "src.services.usage.service.UsageService.update_settled_billing",
         update_settled,
     )
 
@@ -169,15 +169,15 @@ async def test_video_finalize_strict_mode_missing_required_marks_failed(
     try:
         config.billing_strict_mode = True
         monkeypatch.setattr(
-            "src.services.task.application.FormulaEngine.evaluate",
+            "src.services.billing.formula_engine.FormulaEngine.evaluate",
             MagicMock(
                 side_effect=BillingIncompleteError(
                     "Missing required dimensions", missing_required=["duration_seconds"]
                 )
             ),
         )
-        app = TaskApplicationService(db)
-        await app.finalize_video_task(task)
+        svc = TaskService(db)
+        await svc.finalize_video_task(task)
     finally:
         config.billing_strict_mode = old