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/ports/llm.py

@@ -16,6 +16,10 @@ class LLMProvider(Protocol):
     ) -> str:
         """Single-turn completion, returns full response text.
 
+        Implementations MUST wrap underlying SDK calls with their own telemetry
+        (e.g. ``langchain_invoke_span``) so callers such as ``LlmGateway`` can
+        invoke ``complete()`` without adding a second observability layer.
+
         ``max_tokens`` when set is passed to the underlying chat API (adapter-specific).
         """