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/service.py

@@ -7,7 +7,11 @@ from app.features.plan.catalog import (
     get_plan_by_type,
     get_plans_for_api,
 )
-from app.features.plan.schemas import CurrentPlanResponse, PlanResponse
+from app.features.plan.schemas import (
+    CurrentPlanResponse,
+    PlanResponse,
+    PlanUsageResponse,
+)
 from app.features.quota.service import QuotaService
 from app.features.user.models import User
 
@@ -32,12 +36,12 @@ class PlanService:
     async def get_current_plan_response(self, user: User) -> CurrentPlanResponse:
         plan = get_plan_by_type(user.subscription_type)
         segment_count, chapter_count = await self._quota.get_usage(user.id)
-        usage = {
-            "conversations": segment_count,
-            "chapters": chapter_count,
-            "max_conversations": plan.max_conversations,
-            "max_chapters": plan.max_chapters,
-        }
+        usage = PlanUsageResponse(
+            conversations=segment_count,
+            chapters=chapter_count,
+            max_conversations=plan.max_conversations,
+            max_chapters=plan.max_chapters,
+        )
         expires_at = None
         if user.subscription_expires_at:
             expires_at = user.subscription_expires_at.isoformat()