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/catalog.py

@@ -3,7 +3,7 @@
 from app.core.config import settings
 from app.features.plan.schemas import PlanResponse
 
-ENABLE_TEST_PLAN = (settings.enable_test_plan or "").lower() in ("1", "true", "yes")
+ENABLE_TEST_PLAN = settings.enable_test_plan
 
 AVAILABLE_PLANS = [
     PlanResponse(