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/agents/chat/personas.py

@@ -7,6 +7,8 @@ from __future__ import annotations
 
 from typing import Final
 
+from app.features.conversation.constants import chat
+
 # Brand / interviewer name — keep aligned with frontend i18n `conversation.agentName`,
 # OpenAPI title, README, and project metadata. zh = 「岁月知己」，en = Life Echo.
 AGENT_NAME_ZH: Final[str] = "岁月知己"
@@ -18,7 +20,7 @@ def agent_name(language: str = "zh") -> str:
     return AGENT_NAME_EN if (language or "zh").strip().lower() == "en" else AGENT_NAME_ZH
 
 
-# 与 settings.chat_interview_persona 及文档保持一致
+# 与 chat.interview_persona 及文档保持一致
 VALID_INTERVIEW_PERSONAS: Final[frozenset[str]] = frozenset(
     {"default", "warm_listener", "curious_guide"}
 )