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>

app/engine/services/rate_limit.py

@@ -39,5 +39,21 @@ def check(
     return True
 
 
+def sweep(now: float | None = None) -> int:
+    """Sweep expired buckets (spec 2026-10-19 §7 memory cap): drop keys whose
+    timestamps have ALL left the window (boundary inclusive, matching check()'s
+    keep predicate ``t > now - WINDOW``); return the number removed.
+
+    Companion to check()'s per-key lazy pruning: keys never revisited (client
+    churning tenant/ip/device_id) would otherwise accumulate forever; called
+    by main.lifespan's periodic task."""
+    now = time.time() if now is None else now
+    dead = [k for k, ts in _buckets.items()
+            if not ts or ts[-1] <= now - WINDOW]
+    for k in dead:
+        del _buckets[k]
+    return len(dead)
+
+
 def _reset_for_test() -> None:
     _buckets.clear()

app/main.py

@@ -1,5 +1,6 @@
-from contextlib import asynccontextmanager
+from contextlib import asynccontextmanager, suppress
 from pathlib import Path
+import asyncio
 import logging
 import os
 
@@ -12,6 +13,8 @@ from starlette.responses import FileResponse, PlainTextResponse
 from starlette.types import Receive, Scope, Send
 
 from app.engine.config import STATIC_DIR, settings
+from app.engine.services import rate_limit
+from app.platform.auth import login_limiter
 from app.platform.db import (
     Tenant,
     bootstrap_superuser,
@@ -141,6 +144,10 @@ async def lifespan(app: FastAPI):
             registry = TenantRegistry.load_from_dir(SEED_DIR, data_dir=TENANTS_DIR)
         app.state.registry = registry
 
+    # 周期清扫任务句柄:启动成功(yield 前)才创建,finally 里按非 None 取消
+    # ——启动半途失败时跳过,不给清理段引入未定义名。
+    _sweep_task: asyncio.Task | None = None
+
     try:
         await ensure_platform_schema(platform_engine)
         # 平台设置快照(Phase 4 Task 5A):schema 就绪后全表读入内存——
@@ -196,11 +203,26 @@ async def lifespan(app: FastAPI):
                     "启动清扫失败,跳过该租户(slug=%s, db=%s)",
                     rt.config.slug, rt.db_path)
 
+        # 铁律(2026-10-19)进程内面: 登录/聊天限流器过期键周期清扫(spec
+        # §7——两处内存态字典只增不删,不被回访的键永生;语义见各 sweep
+        # 注记)。300s 量级足够:清速远高于键积压速度,首步 sleep 不占启动。
+        async def _limiter_sweep_loop() -> None:
+            while True:
+                await asyncio.sleep(300)
+                login_limiter.sweep()
+                rate_limit.sweep()
+
+        _sweep_task = asyncio.create_task(_limiter_sweep_loop())
+
         yield
     finally:
         # 幂等于"更少的引擎":启动半途失败也把已建好的 engine 关掉
         # (Windows 文件锁: 不关则库文件删不掉)。platform_engine 含预置的
         # 测试引擎——dispose 后 engine 对象仍可重连,测试侧重置引用即可。
+        if _sweep_task is not None:
+            _sweep_task.cancel()
+            with suppress(asyncio.CancelledError):
+                await _sweep_task    # 等 cancel 落地:不留 pending task 给循环收尾
         await registry.dispose_all()
         await platform_engine.dispose()
 

app/platform/auth.py

@@ -213,6 +213,19 @@ class LoginLimiter:
         else:
             self._fails.pop(key, None)
 
+    def sweep(self, now: float | None = None) -> int:
+        """清扫过期键(spec 2026-10-19 §7——内存上限铁律):删「全部时间戳
+        均已出窗」的键,返回清除数。与 _prune 的单键懒清理互补——allow/
+        record_fail 只顺手清被回访的键,失败一次即消失的键(扫号换 IP/
+        用户名)永不被回访,靠本方法全表兜底;由 main.lifespan 周期任务
+        调用。时间戳按记录序追加,ts[-1] 即最新——最新出窗则全键出窗。"""
+        now = time.time() if now is None else now
+        dead = [k for k, ts in self._fails.items()
+                if not ts or ts[-1] <= now - self.window_seconds]
+        for k in dead:
+            del self._fails[k]
+        return len(dead)
+
 
 # 模块级单例: 路由与依赖共用同一份进程内存态(同 rate_limit._buckets 模式)
 token_store = TokenStore()