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/evaluation/gating_service.py

@@ -7,6 +7,7 @@ from typing import Any
 
 from app.core.config import settings
 from app.features.evaluation.models import EvalCase, EvalRun
+from app.features.evaluation.constants import eval_cfg
 
 
 @dataclass
@@ -28,7 +29,7 @@ def compute_gate(
     thr = (
         regression_threshold
         if regression_threshold is not None
-        else settings.eval_gate_protected_regression_threshold
+        else eval_cfg.gate_protected_regression_threshold
     )
     by_case: dict[str, dict[str, EvalRun]] = {}
     for r in runs: