fix: TTS 合成接口失败时返回 201 + failed 状态，不抛 500

与音色克隆接口保持一致：
- 路由层增加 try/except 兜底，捕获 workflow 未处理的意外异常（DB错误等）
- CosyVoice 失败时标记 job 为 failed，返回 201 + failed 状态任务记录
- Celery 调度失败时也标记 job 为 failed，不抛 500
- 新增 10 个单元测试覆盖各种失败场景

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/app/api/routes/tts.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import logging
 from typing import Optional
 
 from app.auth import AuthenticatedUser, get_current_user
@@ -30,6 +31,8 @@ from packages.application.tts_job.use_cases import (
 )
 from packages.application.tts_job.workflow import TTSWorkflowService
 
+logger = logging.getLogger(__name__)
+
 router = APIRouter()
 
 
@@ -74,6 +77,7 @@ def synthesize(
     """发起 TTS 合成任务。
 
     创建 TTS 任务 → 提交 CosyVoice 合成 → 触发 Celery 异步轮询。
+    与音色克隆接口保持一致：CosyVoice 失败时不抛 500，而是返回 201 + failed 状态任务记录。
     """
     user_id = authenticated_user.user.id
     use_case = CreateTTSJobUseCase(repository)
@@ -91,7 +95,18 @@ def synthesize(
         repository=repository,
         cosyvoice_service=cosyvoice_service,
     )
-    job = workflow.start_synthesis(job.id)
+
+    try:
+        job = workflow.start_synthesis(job.id)
+    except Exception as e:
+        # 兜底：workflow 内部已捕获 CosyVoiceError / ValueError，
+        # 但 DB 异常、网络异常等意外错误可能逃逸。
+        # 与音色克隆接口保持一致：标记 failed，返回 201，不抛 500。
+        logger.error(f"TTS 合成异常: job_id={job.id}, error={e}", exc_info=True)
+        try:
+            job = workflow.process_synthesis_failure(job.id, str(e))
+        except Exception as inner_e:
+            logger.error(f"标记 TTS job 失败时出错: job_id={job.id}, error={inner_e}")
 
     # 若任务处于 processing 状态（异步模式），触发 Celery 后台轮询
     if job.status.value == "processing":
@@ -103,7 +118,10 @@ def synthesize(
                 process_tts_synthesis.delay(job.id)
             except Exception as e:
                 # Celery 调度失败，标记 job 为 failed
-                workflow.process_synthesis_failure(job.id, f"Celery 任务调度失败: {e}")
+                try:
+                    workflow.process_synthesis_failure(job.id, f"Celery 任务调度失败: {e}")
+                except Exception as inner_e:
+                    logger.error(f"Celery 调度后标记失败时出错: job_id={job.id}, error={inner_e}")
 
     return TTSSynthesizeResponse(
         job_id=job.id,