feat(domains): identity, billing, task services and domain placeholders

Billing implements reserve -> commit -> refund against bl_quota and bl_ledger
with row-level locking and idempotency keys; the refund commits on its own
transaction so a failed request cannot silently keep a user's quota. Task
implements the unified task centre over the state machine. Identity issues and
resolves users. Remaining domains are placeholders whose READMEs state what
belongs there and what must not.

packages/notify/README.md

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+# `packages/notify/` — 通知（占位）
+
+**职责**：推送（华为 Push Kit）、应用内消息、推送 Token 管理。
+
+## 待实现
+
+- FR-TASK-04：任务完成推送，到达率 ≥ 90%
+- 表：`id_device.push_token` 的管理已在 identity 域预留
+
+## 约束
+
+- **推送文案不得含敏感结论**（FR-TASK-04）。不要在通知里写诊断结果。
+- 推送失败不可阻断主流程：应用内任务中心轮询是兜底（架构 §2 降级表）。

packages/notify/__init__.py

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+"""notify 域：推送、应用内消息、Token 管理。占位。"""