feat(rate-limit): 实现分层 RPM 限速，支持系统默认/用户/独立Key三级配置

- 新增用户级 rate_limit 字段，支持系统默认/用户自定义/不限制三种模式
- 独立 Key 的 rate_limit 语义调整：null=跟随系统默认，0=不限制，>0=自定义
- 实现 UserRpmLimiter 基于 Redis sliding window 的 RPM 限速引擎
- Pipeline 请求流程集成用户级 RPM 检查
- 管理后台和用户面板新增 RPM 限速配置与实时状态查看
- 系统设置新增全局默认 RPM 配置项
- 迁移脚本回填现有 API Key 的 rate_limit 默认值
- 新增用户/Key RPM 状态监控 API 和前端展示

Closes #231

Co-authored-by: LewisPen <LewisPen@nyadoo.com>

tests/services/test_user_rpm_limiter.py

@@ -0,0 +1,148 @@
+from __future__ import annotations
+
+from collections.abc import AsyncGenerator
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from src.services.rate_limit.user_rpm_limiter import RpmCheckResult, UserRpmLimiter
+
+
+@pytest.fixture
+async def limiter(monkeypatch: pytest.MonkeyPatch) -> AsyncGenerator[UserRpmLimiter]:
+    limiter = UserRpmLimiter()
+    limiter._redis = None
+    limiter._memory_counts.clear()
+    await limiter.close()
+    # 阻止 initialize() 连接真实 Redis，确保纯内存模式
+    monkeypatch.setattr(limiter, "initialize", AsyncMock())
+    yield limiter
+    limiter._memory_counts.clear()
+    limiter._redis = None
+    await limiter.close()
+
+
+@pytest.mark.asyncio
+async def test_check_and_consume_enforces_user_scope_in_memory(
+    limiter: UserRpmLimiter,
+) -> None:
+    user_key = limiter.get_user_rpm_key("user-1")
+    key_key = limiter.get_key_rpm_key("key-1")
+
+    first = await limiter.check_and_consume(
+        user_rpm_key=user_key,
+        user_rpm_limit=1,
+        key_rpm_key=key_key,
+        key_rpm_limit=0,
+    )
+    second = await limiter.check_and_consume(
+        user_rpm_key=user_key,
+        user_rpm_limit=1,
+        key_rpm_key=key_key,
+        key_rpm_limit=0,
+    )
+
+    assert first == RpmCheckResult(allowed=True, remaining=0)
+    assert second.allowed is False
+    assert second.scope == "user"
+    assert second.limit == 1
+    assert second.remaining == 0
+
+
+@pytest.mark.asyncio
+async def test_check_and_consume_enforces_key_scope_in_memory(
+    limiter: UserRpmLimiter,
+) -> None:
+    user_key = limiter.get_user_rpm_key("user-1")
+    key_key = limiter.get_key_rpm_key("key-1")
+
+    first = await limiter.check_and_consume(
+        user_rpm_key=user_key,
+        user_rpm_limit=3,
+        key_rpm_key=key_key,
+        key_rpm_limit=1,
+    )
+    second = await limiter.check_and_consume(
+        user_rpm_key=user_key,
+        user_rpm_limit=3,
+        key_rpm_key=key_key,
+        key_rpm_limit=1,
+    )
+
+    assert first.allowed is True
+    assert second.allowed is False
+    assert second.scope == "key"
+    assert second.limit == 1
+
+
+@pytest.mark.asyncio
+async def test_check_and_consume_redis_failure_falls_back_to_memory_when_fail_close(
+    monkeypatch: pytest.MonkeyPatch,
+    limiter: UserRpmLimiter,
+) -> None:
+    fake_redis = MagicMock()
+    fake_redis.eval = AsyncMock(side_effect=RuntimeError("redis down"))
+    limiter._redis = fake_redis
+
+    monkeypatch.setattr(
+        "src.services.rate_limit.user_rpm_limiter.config.rate_limit_fail_open", False
+    )
+
+    user_key = limiter.get_user_rpm_key("user-1")
+    key_key = limiter.get_key_rpm_key("key-1")
+
+    first = await limiter.check_and_consume(
+        user_rpm_key=user_key,
+        user_rpm_limit=1,
+        key_rpm_key=key_key,
+        key_rpm_limit=0,
+    )
+    second = await limiter.check_and_consume(
+        user_rpm_key=user_key,
+        user_rpm_limit=1,
+        key_rpm_key=key_key,
+        key_rpm_limit=0,
+    )
+
+    assert first.allowed is True
+    assert second.allowed is False
+    assert second.scope == "user"
+
+
+@pytest.mark.asyncio
+async def test_check_and_consume_redis_failure_fail_open_allows_request(
+    monkeypatch: pytest.MonkeyPatch,
+    limiter: UserRpmLimiter,
+) -> None:
+    fake_redis = MagicMock()
+    fake_redis.eval = AsyncMock(side_effect=RuntimeError("redis down"))
+    limiter._redis = fake_redis
+
+    monkeypatch.setattr(
+        "src.services.rate_limit.user_rpm_limiter.config.rate_limit_fail_open", True
+    )
+
+    result = await limiter.check_and_consume(
+        user_rpm_key=limiter.get_user_rpm_key("user-1"),
+        user_rpm_limit=1,
+        key_rpm_key=limiter.get_key_rpm_key("key-1"),
+        key_rpm_limit=0,
+    )
+
+    assert result.allowed is True
+
+
+@pytest.mark.asyncio
+async def test_check_and_consume_skips_when_all_limits_are_zero(
+    limiter: UserRpmLimiter,
+) -> None:
+    result = await limiter.check_and_consume(
+        user_rpm_key=limiter.get_user_rpm_key("user-1"),
+        user_rpm_limit=0,
+        key_rpm_key=limiter.get_key_rpm_key("key-1"),
+        key_rpm_limit=0,
+    )
+
+    assert result.allowed is True
+    assert result.scope is None
+    assert limiter._memory_counts == {}