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/retrieval/__init__.py

@@ -0,0 +1 @@
+"""混合检索：硬过滤 → 规则命中 → 向量召回 → BM25 → 重排 → 解释路径。占位。"""