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_recompose_retry_policy.py

@@ -8,7 +8,7 @@ from unittest.mock import MagicMock, patch
 import pytest
 from celery.exceptions import Retry
 
-from app.core.config import settings
+from app.features.memoir.constants import memoir
 from app.tasks.chapter_compose_tasks import recompose_chapter
 
 
@@ -29,7 +29,7 @@ def test_recompose_retries_when_lock_busy_and_flag_on(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
     monkeypatch.setattr(
-        settings, "memoir_recompose_retry_on_lock_contention", True, raising=False
+        memoir, "recompose_retry_on_lock_contention", True, raising=False
     )
     session = MagicMock()
     ch = MagicMock()
@@ -62,7 +62,7 @@ def test_recompose_skips_when_lock_busy_and_flag_off(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
     monkeypatch.setattr(
-        settings, "memoir_recompose_retry_on_lock_contention", False, raising=False
+        memoir, "recompose_retry_on_lock_contention", False, raising=False
     )
     session = MagicMock()
     ch = MagicMock()