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/compliance/service.py

@@ -0,0 +1,69 @@
+"""统一合规出口 — interface only (架构文档 §7.5, CR-05, CR-06).
+
+**所有对外生成物必须经过 ``publish_asset()``，没有例外。** The signature is
+fixed here so that business code written against it does not have to change when
+the implementation lands (T-P1-22 / T-P1-23).
+
+Nothing in this package may import a business domain — it must be safe to call
+from anywhere (架构文档 §4.2 禁止的依赖). import-linter enforces that.
+
+Not implemented on purpose; implementing it half-way is worse than not having
+it, because a half-working moderation call looks like a working one:
+  1. 内容审核 (fail-close: 审核服务不可用则拒绝发布) — 腾讯云 IMS/TMS
+  2. 27 位内容编号生成与 ``cp_content_registry`` 登记
+  3. 显式标识: 图片可见「AI 生成」水印 / 文本提示语
+  4. 隐式标识: 文件元数据 (属性信息 + 服务提供者编码 + 内容编号)
+  5. 上传目标 COS 桶并返回签名地址
+
+必须专门测试的点 (§7.5): 图片经 CDN、缩略图处理、客户端保存与再次分享后，
+元数据是否仍然存在。很多图片处理链路会剥离 EXIF/XMP。
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Literal
+
+ContentType = Literal["image", "text", "pdf"]
+ModerationSuggestion = Literal["pass", "review", "block"]
+
+_NOT_IMPLEMENTED = (
+    "compliance is a structural placeholder (T-P1-22 / T-P1-23). "
+    "No generated content may be released to users until 审核 + 水印 + 元数据 + "
+    "内容编号登记 are all in place — CR-05/CR-06 are 不可裁剪."
+)
+
+
+@dataclass(frozen=True, slots=True)
+class ContentRef:
+    ref_type: str
+    ref_id: str
+
+
+@dataclass(frozen=True, slots=True)
+class ModelInfo:
+    provider: str
+    model: str
+    prompt_id: str | None = None
+    prompt_version: str | None = None
+
+
+@dataclass(frozen=True, slots=True)
+class PublishedAsset:
+    content_code: str
+    url: str
+    moderation: ModerationSuggestion
+
+
+async def moderate(text: str) -> ModerationSuggestion:
+    """审核不可降级：服务不可用时按拦截处理 (fail-close, 架构文档 §11.4)."""
+    raise NotImplementedError(_NOT_IMPLEMENTED)
+
+
+async def publish_asset(
+    raw: bytes | str,
+    content_type: ContentType,
+    ref: ContentRef,
+    model_info: ModelInfo,
+) -> PublishedAsset:
+    raise NotImplementedError(_NOT_IMPLEMENTED)