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/billing/repo.py

@@ -0,0 +1,79 @@
+"""Data access for the billing domain. **Only this module may touch bl_* tables.**
+
+架构文档 §3.2 硬性规则 1: 每个域只能通过对方暴露的 service 接口调用，禁止直接
+查对方的表。import-linter enforces it (contract "no cross-domain repository access").
+"""
+
+from __future__ import annotations
+
+from sqlalchemy import select
+from sqlalchemy.dialects.postgresql import insert as pg_insert
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from packages.shared.models import Ledger, Quota, QuotaRule
+
+
+async def get_rule(session: AsyncSession, quota_key: str) -> QuotaRule | None:
+    return await session.get(QuotaRule, quota_key)
+
+
+async def lock_or_create_quota(
+    session: AsyncSession,
+    *,
+    user_id: int,
+    quota_key: str,
+    period_key: str,
+    limit_value: int,
+) -> Quota:
+    """Get the period row with a row lock, creating it on first use.
+
+    ``ON CONFLICT DO NOTHING`` + ``SELECT ... FOR UPDATE`` makes concurrent first
+    requests for the same period safe without an advisory lock.
+    """
+    await session.execute(
+        pg_insert(Quota)
+        .values(
+            user_id=user_id,
+            quota_key=quota_key,
+            period_key=period_key,
+            used=0,
+            limit_value=limit_value,
+            bonus_value=0,
+        )
+        .on_conflict_do_nothing(index_elements=["user_id", "quota_key", "period_key"])
+    )
+    result = await session.execute(
+        select(Quota)
+        .where(
+            Quota.user_id == user_id,
+            Quota.quota_key == quota_key,
+            Quota.period_key == period_key,
+        )
+        .with_for_update()
+    )
+    row = result.scalar_one()
+    return row
+
+
+async def get_ledger_by_idempotency_key(session: AsyncSession, key: str) -> Ledger | None:
+    result = await session.execute(select(Ledger).where(Ledger.idempotency_key == key))
+    return result.scalar_one_or_none()
+
+
+async def get_ledger(session: AsyncSession, tx_id: int) -> Ledger | None:
+    return await session.get(Ledger, tx_id)
+
+
+async def list_ledger(session: AsyncSession, user_id: int, limit: int = 50) -> list[Ledger]:
+    result = await session.execute(
+        select(Ledger)
+        .where(Ledger.user_id == user_id)
+        .order_by(Ledger.created_at.desc(), Ledger.tx_id.desc())
+        .limit(limit)
+    )
+    return list(result.scalars().all())
+
+
+def add_ledger(session: AsyncSession, ledger: Ledger) -> Ledger:
+    session.add(ledger)
+    return ledger