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/orchestrator.py

@@ -29,6 +29,7 @@ from app.core.agent_logging import agent_span, agent_summary_enabled, log_agent_
 from app.core.config import settings
 from app.core.logging import get_logger
 from app.features.conversation.models import Segment
+from app.features.memoir.constants import memoir
 
 logger = get_logger(__name__)
 
@@ -90,7 +91,7 @@ class MemoirOrchestrator:
         use_batch = (
             bool(segments)
             and classify_extract_llm is not None
-            and settings.memoir_phase1_batch_llm_enabled
+            and memoir.phase1_batch_llm_enabled
         )
         if use_batch:
             try:
@@ -204,7 +205,7 @@ class MemoirOrchestrator:
             segments,
             state,
             classify_extract_llm,
-            chunk_size=int(settings.memoir_phase1_batch_llm_chunk_size),
+            chunk_size=int(memoir.phase1_batch_llm_chunk_size),
             on_chunk=on_phase1_chunk,
             language=language,
         )