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/plan/schemas.py

@@ -1,5 +1,3 @@
-from typing import List, Optional
-
 from pydantic import BaseModel
 
 
@@ -9,17 +7,24 @@ class PlanResponse(BaseModel):
     display_name: str
     price: float
     currency: str
-    features: List[str]
-    max_conversations: Optional[int] = None
-    max_chapters: Optional[int] = None
-    max_words: Optional[int] = None
+    features: list[str]
+    max_conversations: int | None = None
+    max_chapters: int | None = None
+    max_words: int | None = None
     is_popular: bool = False
 
 
+class PlanUsageResponse(BaseModel):
+    conversations: int
+    chapters: int
+    max_conversations: int | None = None
+    max_chapters: int | None = None
+
+
 class CurrentPlanResponse(BaseModel):
     plan_id: str
     plan_name: str
     subscription_type: str
-    expires_at: Optional[str] = None
-    features: List[str]
-    usage: dict
+    expires_at: str | None = None
+    features: list[str]
+    usage: PlanUsageResponse