feat(routes): 迁移下载/批量打包/播客下载（账本事务）+ 修密钥绑定缺陷

迁移端点：
- POST /api/v1/download（单曲：file 后台任务 / json 同步交付+结算）
- POST /api/v1/download/batch_zip（按去重歌曲数计费）
- POST /api/v1/podcast/download、/podcast/download/batch_zip
- 新增 web/billing.py（预扣+开账同事务 / 只退 open 行 / rename 承接）
  与 web/tasks_registry.py（任务登记/容量闸/账本撞名重试，与 main 同口径）
- worker 仍复用 main 既有实现（无 flask 依赖），账本与交付语义不变

修复我引入的真实缺陷：
- download 参数校验分支（ID/音质/格式非法）漏退款 → 预扣泄漏。改为统一 _fail() 退款
- web/routers/{auth,register}.py 在导入期绑定 deps.secret_key，导致 patch web.deps.secret_key
  无效、且测试顺序相关。改为调用期 deps.secret_key() 解析

测试：tests/test_download_billing.py 7 项沙箱用例如实断言账本守恒
（预扣必退、无 open 行残留、次数不足不建任务）；
全套 pytest 69/69（顺序无关）；ruff All checks passed；pyright 0 errors；奇偶 36/36。

web/tasks_registry.py

@@ -0,0 +1,79 @@
+# -*- coding: utf-8 -*-
+"""下载/播客任务登记（FastAPI 栈）——镜像 main._register_download_task。
+
+与 Flask 版的唯一差异：不依赖 flask.g，请求级账本键由 Billing 显式携带。
+其余（归属、容量闸、task_id 白名单与碰撞换新、账本承接与撞名重试）逐条对齐。
+"""
+import logging
+import re
+import uuid
+from collections.abc import Callable
+from typing import Any
+
+from core.config import MAX_ACTIVE_TASKS_PER_USER, MAX_TERMINAL_TASKS, MAX_TOTAL_TASKS
+from core.state import DOWNLOAD_TASKS, DOWNLOAD_TASKS_LOCK
+from web.billing import Billing
+
+logger = logging.getLogger('web.tasks')
+
+_TASK_ID_RE = re.compile(r'^[A-Za-z0-9_-]{1,32}$')
+
+
+async def register_download_task(billing: Billing, task: dict[str, Any],
+                                 task_id_hint: str = '') -> str | None:
+    """任务登记：失败返回错误描述，成功返回 None。task['task_id'] 为最终 id。"""
+    user_id = billing.user.id
+    task['user_id'] = user_id
+    task['billing_reserved'] = billing.need
+
+    with DOWNLOAD_TASKS_LOCK:
+        tid = (task_id_hint or '').strip() or uuid.uuid4().hex[:12]
+        if not _TASK_ID_RE.match(tid):
+            tid = uuid.uuid4().hex[:12]
+        while tid in DOWNLOAD_TASKS:
+            tid = uuid.uuid4().hex[:12]
+        active = sum(1 for t in DOWNLOAD_TASKS.values()
+                     if t.get('user_id') == user_id
+                     and t.get('status') in ('queued', 'running', 'paused'))
+        if active >= MAX_ACTIVE_TASKS_PER_USER:
+            return f'您有 {active} 个任务正在进行，请等待完成或取消后再提交'
+        active_total = sum(1 for t in DOWNLOAD_TASKS.values()
+                           if t.get('status') in ('queued', 'running', 'paused'))
+        if active_total >= MAX_TOTAL_TASKS:
+            return '当前任务队列已满，请稍后再试'
+        if len(DOWNLOAD_TASKS) - active_total >= MAX_TERMINAL_TASKS:
+            return '当前任务队列已满，请稍后再试'
+        task['task_id'] = tid
+        DOWNLOAD_TASKS[tid] = task
+
+    # 账本承接（撞名重试，与 Flask 同口径）
+    try:
+        renamed = await billing.rename_to(tid)
+        retries = 0
+        while not renamed and retries < 5:
+            retries += 1
+            with DOWNLOAD_TASKS_LOCK:
+                DOWNLOAD_TASKS.pop(tid, None)
+                tid = uuid.uuid4().hex[:12]
+                while tid in DOWNLOAD_TASKS:
+                    tid = uuid.uuid4().hex[:12]
+                task['task_id'] = tid
+                DOWNLOAD_TASKS[tid] = task
+            renamed = await billing.rename_to(tid)
+        if not renamed:
+            raise RuntimeError('任务账本键撞名，请重新提交')
+        billing.ledger_key = tid
+    except Exception:
+        with DOWNLOAD_TASKS_LOCK:
+            if DOWNLOAD_TASKS.get(tid) is task:
+                DOWNLOAD_TASKS.pop(tid, None)
+        raise
+    return None
+
+
+async def start_task(task_id: str, worker: Callable, args: tuple = ()) -> str | None:
+    """启动 worker 线程（复用 main._start_task_thread 的实现，失败收敛终态并退费）。"""
+    import asyncio
+
+    import main as legacy
+    return await asyncio.to_thread(legacy._start_task_thread, task_id, worker, args)