feat(api): vertical slice POST /v1/agent/chat and the Celery ping path

One request exercises every layer: JWT auth, quota reservation, a versioned
prompt through the LLM gateway, a cost row, quota commit or refund, and the
unified envelope. The trivial ping task proves the queue routing and the
tk_task transitions before any real long-running task is written.

Routes are mounted at the /api/v1 base path of 架构文档 §9.1; /v1/* is
rewritten to it at the ASGI layer so both spellings work from one route table.

apps/api/__init__.py

@@ -0,0 +1 @@
+"""FastAPI 应用（架构文档 §3.1 L3 接入编排层）。"""