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/tests/test_story_route_payload.py

@@ -13,6 +13,7 @@ from app.agents.memoir.story_route_payload import (
     _truncate_body_for_route,
 )
 from app.core.config import Settings
+from app.features.story.constants import story
 
 
 def _story(**kwargs):
@@ -102,8 +103,8 @@ def test_long_body_uses_head_tail():
 
 def test_total_budget_downgrades_tail_rows(monkeypatch):
     settings = Settings()
-    monkeypatch.setattr(settings, "story_route_candidate_total_max_chars", 800)
-    monkeypatch.setattr(settings, "story_route_index_preview_chars", 40)
+    monkeypatch.setattr(story, "route_candidate_total_max_chars", 800)
+    monkeypatch.setattr(story, "route_index_preview_chars", 40)
     stories = [
         _story(
             id="1",