refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/app/tasks/story_title_tasks.py

@@ -4,14 +4,14 @@ import time
 
 from celery import shared_task
 
-from app.core.db import get_sync_db
+from app.core.db import get_sync_db, transactional_sync
 from app.core.llm_gateway import LlmGateway, LlmUseCase
 from app.core.logging import get_logger
 
 logger = get_logger(__name__)
 
 
-@shared_task(bind=True, max_retries=2, default_retry_delay=15)
+@shared_task(bind=True, max_retries=2, default_retry_delay=15, ignore_result=True)
 def generate_story_title_after_create(
     self,
     story_id: str,
@@ -128,8 +128,8 @@ def generate_story_title_after_create(
                     ms,
                 )
                 return {"status": "skip_placeholder"}
-            st.title = new_title
-            db.commit()
+            with transactional_sync(db):
+                st.title = new_title
             ms = (time.perf_counter() - t0) * 1000
             logger.info(
                 "event=story_title_task_done story_id={} user_id={} duration_ms={:.1f} "