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/memory/models.py

@@ -33,6 +33,7 @@ class MemorySource(Base):
     enrichment_status = Column(String, default="pending")
     enrichment_error = Column(Text, nullable=True)
     conversation_id = Column(String, ForeignKey("conversations.id"), nullable=True)
+    segment_id = Column(String, ForeignKey("segments.id", ondelete="SET NULL"), nullable=True)
     lineage_json = Column(JSON, nullable=True)
     primary_user_message_id = Column(String, nullable=True)
     created_at = Column(DateTime(timezone=True), default=utc_now)