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/worker/celery_app.py

@@ -0,0 +1,54 @@
+"""Celery application with the four queues from TD-11 / 架构文档 §3.1.
+
+    gpu   换装、发型、抠图 —— 并发上限由 GPU 容量决定，队列即容量 (§7.3)
+    cpu   报告视觉分析、渲染
+    io    外部 API 调用、推送、上传
+    beat  定时任务（单实例，防重）
+
+Queue isolation is the point: 换装排队不能把报告生成拖慢。
+"""
+
+from __future__ import annotations
+
+from celery import Celery
+from kombu import Queue
+
+from packages.shared.config import get_settings
+from packages.shared.logging import configure_logging
+
+settings = get_settings()
+configure_logging(
+    level=settings.log_level,
+    json_output=settings.log_json,
+    extra_redact_keys=settings.extra_redact_keys,
+)
+
+celery_app = Celery(
+    "meida",
+    broker=settings.celery_broker_url,
+    backend=settings.celery_result_backend,
+    include=["apps.worker.tasks"],
+)
+
+celery_app.conf.update(
+    task_default_queue="cpu",
+    task_queues=(
+        Queue("gpu"),
+        Queue("cpu"),
+        Queue("io"),
+        Queue("beat"),
+    ),
+    task_routes={
+        "meida.tryon.*": {"queue": "gpu"},
+        "meida.matting.*": {"queue": "gpu"},
+        "meida.report.*": {"queue": "cpu"},
+        "meida.notify.*": {"queue": "io"},
+    },
+    task_acks_late=True,
+    task_reject_on_worker_lost=True,
+    task_track_started=True,
+    worker_prefetch_multiplier=1,
+    broker_connection_retry_on_startup=True,
+    timezone="Asia/Shanghai",
+    result_expires=86400,
+)