feat(limits): 限流器过期键周期清扫（内存上限铁律）

LoginLimiter._fails 与 rate_limit._buckets 只增不删——懒清理只覆盖被
回访的键，扫号换 IP/设备轮换产生的键永不被回访 → 进程长跑缓慢累积。
两处各加 sweep()（全表清「全部时间戳均已出窗」的键，窗沿 <= 与既有
保留判据 t > now - W 同沿），main.lifespan 起周期任务（300s loop）
启动创建/关停取消（cancel 后 await 落地，不留 pending task）。

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/test_limiter_sweep.py

@@ -0,0 +1,95 @@
+"""limiter 周期清扫(spec 2026-10-19 §7——日志/内存上限铁律的进程内面)。
+
+两处限流内存态只增不删的兜底:
+- app.platform.auth.LoginLimiter._fails: (username, ip) → list[float]——
+  allow/record_fail 只顺手 _prune 被访问的键;失败一次即消失的键(扫号
+  换 IP/换用户名)永不被回访 → 永久滞留。
+- app.engine.services.rate_limit._buckets: (tenant, ip, device_id) →
+  list[float]——check() 同款单键懒清,键随客户端轮换只增不减。
+sweep() 全表清「全部时间戳均已出窗」的键,由 main.lifespan 的周期任务
+(300s sleep loop)调用。时间推进手法照 test_auth 既有 lockout 用例:
+直改内部时间戳,免真实等待。
+"""
+import asyncio
+import time
+
+from tests.conftest import (
+    JIANCAI_HOST,
+    SEED_DIR,
+    app_test_client,
+    build_app,
+)
+
+
+class TestLoginLimiterSweep:
+    def test_清过期保活跃(self):
+        from app.platform.auth import LoginLimiter
+        limiter = LoginLimiter(window_seconds=900.0, max_fails=5)
+        limiter.record_fail("u", "1.1.1.1")
+        limiter.record_fail("u", "2.2.2.2")
+        limiter._fails[("u", "1.1.1.1")] = [0.0]      # 远古 → 待清
+        n = limiter.sweep(now=time.time())
+        assert n == 1
+        assert ("u", "1.1.1.1") not in limiter._fails
+        assert ("u", "2.2.2.2") in limiter._fails
+
+    def test_全活跃时清零(self):
+        from app.platform.auth import LoginLimiter
+        limiter = LoginLimiter(window_seconds=900.0)
+        limiter.record_fail("u", "1.1.1.1")
+        assert limiter.sweep(now=time.time()) == 0
+        assert ("u", "1.1.1.1") in limiter._fails
+
+    def test_窗沿时间戳算过期_空键也清(self):
+        # ts[-1] <= now - window 与 _prune 的保留判据 t > now - window 同沿
+        from app.platform.auth import LoginLimiter
+        limiter = LoginLimiter(window_seconds=100.0)
+        now = time.time()
+        limiter._fails[("u", "1.1.1.1")] = [now - 100.0]   # 恰在窗沿 → 出窗
+        limiter._fails[("u", "2.2.2.2")] = []              # 空值键 → 待清
+        assert limiter.sweep(now=now) == 2
+        assert not list(limiter._fails)                    # 全清空
+
+
+class TestRateLimitSweep:
+    def test_同款语义(self):
+        from app.engine.services import rate_limit
+        try:
+            now = time.time()
+            rate_limit._buckets[("a", "1.1.1.1", "")] = [0.0]   # 远古
+            rate_limit._buckets[("b", "2.2.2.2", "")] = [now]   # 活跃
+            rate_limit._buckets[("c", "3.3.3.3", "")] = []      # 空值键
+            rate_limit._buckets[("d", "4.4.4.4", "")] = [
+                now - rate_limit.WINDOW]                        # 恰在窗沿
+            n = rate_limit.sweep(now=now)
+            assert n == 3
+            assert set(rate_limit._buckets) == {("b", "2.2.2.2", "")}
+        finally:
+            rate_limit._buckets.clear()   # 模块级状态:清现场(conftest 亦兜底)
+
+
+class TestLifespanSweepTask:
+    def test_注册与取消(self, tmp_path, monkeypatch):
+        """lifespan 起周期清扫任务,关停时取消。
+
+        手法: monkeypatch asyncio.create_task 包一层记录真实 task(行为
+        不变);TestClient with 进出 = lifespan 启停。不真等 300s——loop
+        首步即 sleep,任务常 pending;取消断言 cancelled/cancelling 任一
+        成立即可(实现侧 cancel 后 await 落地,常态是 cancelled)。"""
+        created = []
+        real_create_task = asyncio.create_task
+
+        def _rec(coro, **kwargs):
+            t = real_create_task(coro, **kwargs)
+            created.append(t)
+            return t
+
+        monkeypatch.setattr(asyncio, "create_task", _rec)
+        a = build_app(SEED_DIR, tmp_path)
+        with app_test_client(a, f"http://{JIANCAI_HOST}", tmp_path):
+            tasks = [t for t in created
+                     if getattr(t.get_coro(), "__name__", "")
+                     == "_limiter_sweep_loop"]
+            assert len(tasks) == 1            # 启动期创建且恰好一份
+            assert not tasks[0].done()        # 300s sleep 中不会自然完成
+        assert tasks[0].cancelled() or tasks[0].cancelling() > 0