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/features/memoir/cover_eligibility.py

@@ -9,6 +9,7 @@ from app.features.memoir.asset_resolver import (
     parse_asset_refs,
     strip_image_placeholders,
 )
+from app.features.memoir.constants import memoir
 from app.features.memoir.memoir_images.schema import (
     IMAGE_STATUS_FAILED,
     IMAGE_STATUS_PENDING,
@@ -61,7 +62,7 @@ def chapter_eligible_for_cover_by_inline_body_image_count(
     chapter: Any, *, markdown: str | None = None
 ) -> bool:
     """正文内 asset:// 数量 ≥ 配置阈值时允许封面；markdown 非 None 时仅用该串计数。"""
-    min_required = int(settings.memoir_min_inline_images_for_chapter_cover)
+    min_required = int(memoir.min_inline_images_for_chapter_cover)
     return count_chapter_inline_body_images(chapter, markdown=markdown) >= min_required