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/memoir/classification_agent.py

@@ -26,6 +26,7 @@ from app.core.config import settings
 from app.core.json_utils import extract_json_payload
 from app.core.llm_call import LLMCallError, llm_json_call
 from app.core.logging import get_logger
+from app.features.memoir.constants import memoir
 
 logger = get_logger(__name__)
 
@@ -144,7 +145,7 @@ class ClassificationAgent:
                     llm,
                     prompt,
                     ClassificationOutput,
-                    max_tokens=settings.memoir_classification_max_tokens,
+                    max_tokens=memoir.classification_max_tokens,
                     agent="ClassificationAgent.classify",
                 )
                 category = _normalize_llm_category(out.category)