feat(billing): task bonus claim endpoint with idempotency and cap

POST /quota/tasks/{task_key}/claim：配置驱动任务注册表、
grant_bonus 纯域函数、ledger 幂等键防重、task_bonus_max 封顶（FR-BILL-03/04）。

实现：packages/domain/billing/quota.py 新增 BonusDecision/grant_bonus（mypy strict，
恰好到顶允许、超一格整体拒绝）、grant_idempotency_key、PERIOD_TYPES；
packages/billing/service.py 新增 claim_task_bonus（幂等快捷路径 → 行锁 → 封顶判定
→ 同事务更新 bonus_granted 并写 system_grant 流水，delta=0 且 balance_after 按
remaining 口径；IntegrityError 并发兜底回滚为 40910）；响应模型 TaskBonusClaimResponse
放新建的 packages/billing/schemas.py；路由挂在 apps/api/routers/quota.py，
rules 由 settings.task_bonus_rules 注入（amount 为占位值，产品定稿后改 env）。
测试：新增单测 11 个（封顶边界、amount<=0、不动 used、hard_cap 独立生效、
默认规则形状与 env 覆盖通路）；新增集成 4 个（领取+重复 40910+纯 DB 流水断言、
五类任务全部到账、未知任务 40410、amount=11>max=10 → 40310 且无新流水）。
门禁全绿：ruff check / ruff format --check / mypy 两段 / lint-imports；
单测 180 passed，集成 7 passed。

偏差说明：1) 任务文档提到的 BillingService._load_rule 实际名为 _require_rule，
   直接复用现有助手；2) packages/billing/repo.py 无需新增函数（现有
   get_ledger_by_idempotency_key/lock_or_create_quota/add_ledger 已覆盖本任务）；
   3) 文档设想的「TASK_BONUS_RULES_JSON 空值容错」经 env 源实测不成立——
   pydantic-settings 对 dict 字段先做 JSON 解码，空白字符串在 Settings 构造时即
   报错（共享校验器/config.py 属禁改，未动）；单测改为覆盖受支持的空配置形态 "{}"。

packages/billing/schemas.py

@@ -0,0 +1,20 @@
+"""billing 域对外响应模型（FR-BILL-03 任务奖励）.
+
+``apps/api/schemas.py`` 是全局共享契约文件，本轮并行开发禁改；billing 新增
+接口的响应模型放本包，由路由层装配进 ``ok(...)`` 信封。字段口径与
+NFR-MAINT-01「契约先行」一致：客户端类型从 OpenAPI 生成。
+"""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field
+
+
+class TaskBonusClaimResponse(BaseModel):
+    """``POST /quota/tasks/{task_key}/claim`` 的 data 载荷（FR-BILL-03）."""
+
+    task_key: str = Field(max_length=64, description="任务标识，如 sign_in")
+    quota_key: str = Field(max_length=64, description="奖励落入的额度种类")
+    amount: int = Field(description="本次发放数额（当前为占位值，正式数值由产品定稿后改配置）")
+    bonus_granted: int = Field(description="本周期累计已获得的任务奖励")
+    remaining: int = Field(description="发放后该额度的剩余次数")

packages/billing/service.py

@@ -11,8 +11,9 @@ from __future__ import annotations
 
 from dataclasses import dataclass
 from datetime import UTC, datetime
-from typing import cast
+from typing import Any, cast
 
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from packages.billing import repo
@@ -49,6 +50,43 @@ class QuotaSnapshot:
     reset_at: datetime | None
 
 
+@dataclass(frozen=True, slots=True)
+class TaskBonusClaim:
+    """一次成功领取的结果（路由层据此组装 TaskBonusClaimResponse）."""
+
+    task_key: str
+    quota_key: str
+    amount: int
+    bonus_granted: int
+    remaining: int
+
+
+def _parse_task_rule(
+    task_key: str, conf: dict[str, Any]
+) -> tuple[str, int, quota_rules.PeriodType]:
+    """解析 ``settings.task_bonus_rules`` 的一条配置.
+
+    坏配置是运维错误（5xxxx），不是用户错误：提前在这里拦下，比让
+    ``period_key`` 在请求路径上抛 ValueError 变成 500 清晰得多。
+    """
+    quota_key = conf.get("quota_key")
+    amount = conf.get("amount")
+    claim_period = conf.get("claim_period")
+    if not isinstance(quota_key, str) or not quota_key:
+        raise AppError(
+            ErrorCode.CONFIG_ERROR, message="任务奖励配置不合法", data={"task_key": task_key}
+        )
+    if not isinstance(amount, int) or isinstance(amount, bool) or amount <= 0:
+        raise AppError(
+            ErrorCode.CONFIG_ERROR, message="任务奖励配置不合法", data={"task_key": task_key}
+        )
+    if not isinstance(claim_period, str) or claim_period not in quota_rules.PERIOD_TYPES:
+        raise AppError(
+            ErrorCode.CONFIG_ERROR, message="任务奖励配置不合法", data={"task_key": task_key}
+        )
+    return quota_key, amount, cast(quota_rules.PeriodType, claim_period)
+
+
 class BillingService:
     def __init__(self, session: AsyncSession, *, tz_offset_hours: int = 8) -> None:
         self._session = session
@@ -263,6 +301,119 @@ class BillingService:
             remaining=new_state.remaining,
         )
 
+    # -- task bonus (FR-BILL-03) --------------------------------------------
+
+    async def claim_task_bonus(
+        self,
+        *,
+        user_id: int,
+        task_key: str,
+        rules: dict[str, dict[str, Any]],
+    ) -> TaskBonusClaim:
+        """领取任务奖励：发放即到账、幂等、封顶（FR-BILL-03）.
+
+        ``rules`` 来自 ``settings.task_bonus_rules``（FR-BILL-04 配置化）：
+        amount 目前是占位值，产品在产品方案 §3.2 定稿后改 env 即生效，
+        不需要改代码。
+        """
+        conf = rules.get(task_key)
+        if conf is None:
+            raise AppError(ErrorCode.TASK_BONUS_UNKNOWN, data={"task_key": task_key})
+        quota_key, amount, claim_period = _parse_task_rule(task_key, conf)
+
+        rule = await self._require_rule(quota_key)
+        quota_period_type = cast(quota_rules.PeriodType, rule.period_type)
+        now = datetime.now(UTC)
+        # 幂等粒度跟任务的 claim_period 走，额度行周期跟 bl_quota_rule 走：
+        # 「每天可领」的任务可以给「按月计量」的额度加成，两套周期不能混。
+        quota_pk = quota_rules.period_key(now, quota_period_type, self._tz_offset)
+        claim_pk = quota_rules.period_key(now, claim_period, self._tz_offset)
+        idem_key = quota_rules.grant_idempotency_key(
+            user_id=user_id, task_key=task_key, claim_period_key=claim_pk
+        )
+
+        # 幂等快捷路径：本周期已发放过就直接拒绝，不进任何写路径。
+        if await repo.get_ledger_by_idempotency_key(self._session, idem_key) is not None:
+            QUOTA_EVENTS.labels(quota_key, "grant", "idempotent_hit").inc()
+            raise AppError(ErrorCode.TASK_BONUS_ALREADY_CLAIMED, data={"task_key": task_key})
+
+        row = await repo.lock_or_create_quota(
+            self._session,
+            user_id=user_id,
+            quota_key=quota_key,
+            period_key=quota_pk,
+            limit_value=rule.base_limit,
+        )
+        state = quota_rules.QuotaState(
+            used=row.used,
+            base_limit=row.limit_value,
+            bonus=row.bonus_granted,
+            hard_cap=rule.hard_cap,
+        )
+        decision = quota_rules.grant_bonus(state, amount, rule.task_bonus_max)
+        if not decision.allowed:
+            # 封顶拒绝：不写任何流水，bonus_granted 保持原值（验收 3）。
+            QUOTA_EVENTS.labels(quota_key, "grant", "capped").inc()
+            raise AppError(
+                ErrorCode.TASK_BONUS_CAP_REACHED,
+                data={
+                    "task_key": task_key,
+                    "bonus_granted": state.bonus,
+                    "task_bonus_max": rule.task_bonus_max,
+                },
+            )
+
+        row.bonus_granted = decision.bonus
+        tx_id = next_id()
+        try:
+            repo.add_ledger(
+                self._session,
+                Ledger(
+                    tx_id=tx_id,
+                    user_id=user_id,
+                    # delta=0：system_grant 不改变 used，只提高有效额度；balance_after
+                    # 仍按 remaining 语义记录 —— 流水可重算的是 used，不是余额本身，
+                    # 与 reserve/commit 的记账口径一致。
+                    delta=0,
+                    tx_type="system_grant",
+                    status="committed",
+                    reason=task_key,
+                    ref_type="task_bonus",
+                    ref_id=task_key,
+                    quota_key=quota_key,
+                    period_key=quota_pk,
+                    balance_after=decision.remaining,
+                    idempotency_key=idem_key,
+                ),
+            )
+            await self._session.flush()
+        except IntegrityError as exc:
+            # 并发兜底：两个请求同时通过上面的快捷检查，idempotency_key 唯一约束
+            # 让后提交者失败。对用户来说结果与快捷路径等价 —— 已领取，而非 500。
+            await self._session.rollback()
+            QUOTA_EVENTS.labels(quota_key, "grant", "idempotent_hit").inc()
+            raise AppError(
+                ErrorCode.TASK_BONUS_ALREADY_CLAIMED, data={"task_key": task_key}
+            ) from exc
+
+        QUOTA_EVENTS.labels(quota_key, "grant", "ok").inc()
+        logger.info(
+            "task_bonus_granted",
+            tx_id=tx_id,
+            task_key=task_key,
+            quota_key=quota_key,
+            amount=amount,
+            bonus=decision.bonus,
+            remaining=decision.remaining,
+        )
+        return TaskBonusClaim(
+            task_key=task_key,
+            quota_key=quota_key,
+            amount=amount,
+            bonus_granted=decision.bonus,
+            remaining=decision.remaining,
+        )
+
     # -- helpers ------------------------------------------------------------
 
     async def _require_rule(self, quota_key: str) -> QuotaRule:

packages/domain/billing/quota.py

@@ -15,11 +15,13 @@ from __future__ import annotations
 
 from dataclasses import dataclass, replace
 from datetime import UTC, datetime, timedelta, timezone
-from typing import Literal
+from typing import Literal, get_args
 
 from packages.domain.errors import QuotaExceededError
 
 PeriodType = Literal["day", "month", "lifetime"]
+#: 合法周期取值集合，供配置校验复用；单一事实来源是上面的 Literal。
+PERIOD_TYPES: frozenset[str] = frozenset(get_args(PeriodType))
 LedgerStatus = Literal["reserved", "committed", "refunded"]
 TxType = Literal["system_grant", "consume", "refund", "purchase", "gift"]
 
@@ -125,6 +127,51 @@ def refund(state: QuotaState, amount: int) -> QuotaState:
     return replace(state, used=max(state.used - amount, 0))
 
 
+@dataclass(frozen=True, slots=True)
+class BonusDecision:
+    """任务奖励发放的封顶判定结果（FR-BILL-03）.
+
+    ``allowed=False`` 时 ``bonus/remaining/effective_limit`` 保持发放前的值：
+    调用方必须整体放弃，不做部分发放 ——「做任务换 N 次」对用户是明确承诺，
+    发一半既说不清也无法用流水对账。
+    """
+
+    allowed: bool
+    amount: int
+    bonus: int
+    remaining: int
+    effective_limit: int
+
+
+def grant_bonus(state: QuotaState, amount: int, task_bonus_max: int) -> BonusDecision:
+    """把任务奖励 ``amount`` 计入 ``state.bonus``，超上限则整体拒绝（FR-BILL-03）.
+
+    - 只动 ``bonus``，不动 ``used``：任务奖励抬高的是额度，不是用量；
+    - ``task_bonus_max`` 来自 ``bl_quota_rule``，是任务奖励独立于 ``hard_cap``
+      的第二道天花板（两者都生效时 effective_limit 取更紧的）；
+    - 边界语义：恰好到顶（bonus + amount == max）允许，超出一格（== max+1）拒绝。
+    """
+    if amount <= 0:
+        msg = "amount must be positive"
+        raise ValueError(msg)
+    if state.bonus + amount > task_bonus_max:
+        return BonusDecision(
+            allowed=False,
+            amount=amount,
+            bonus=state.bonus,
+            remaining=state.remaining,
+            effective_limit=state.effective_limit,
+        )
+    granted = replace(state, bonus=state.bonus + amount)
+    return BonusDecision(
+        allowed=True,
+        amount=amount,
+        bonus=granted.bonus,
+        remaining=granted.remaining,
+        effective_limit=granted.effective_limit,
+    )
+
+
 def balance_after(state: QuotaState) -> int:
     """Value written to ``bl_ledger.balance_after``.
 
@@ -139,6 +186,15 @@ def reserve_idempotency_key(*, user_id: int, quota_key: str, request_key: str) -
     return f"rsv:{user_id}:{quota_key}:{request_key}"
 
 
+def grant_idempotency_key(*, user_id: int, task_key: str, claim_period_key: str) -> str:
+    """FR-BILL-03 发放幂等：同一任务同一周期派生同一个键.
+
+    周期取任务的 ``claim_period``（不是额度行的周期）——「每天可领」的月度
+    额度任务，幂等粒度必须跟任务承诺走。
+    """
+    return f"grant:{user_id}:{task_key}:{claim_period_key}"
+
+
 def refund_idempotency_key(reserve_key: str) -> str:
     """FR-BILL-05: 退还必须幂等。Derived, so a retry cannot double-refund."""
     return f"rfd:{reserve_key}"