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/api/test_admin_user_routes.py

@@ -49,6 +49,7 @@ def test_list_users_uses_wallet_batch_lookup(monkeypatch: pytest.MonkeyPatch) ->
             allowed_providers=None,
             allowed_api_formats=None,
             allowed_models=None,
+            rate_limit=None,
             is_active=True,
             created_at=now,
             updated_at=now,
@@ -62,6 +63,7 @@ def test_list_users_uses_wallet_batch_lookup(monkeypatch: pytest.MonkeyPatch) ->
             allowed_providers=None,
             allowed_api_formats=None,
             allowed_models=None,
+            rate_limit=None,
             is_active=True,
             created_at=now,
             updated_at=None,
@@ -101,24 +103,14 @@ async def test_create_user_adapter_preserves_empty_restriction_lists(
     db = MagicMock()
     captured: dict[str, Any] = {}
 
-    def _create_user(**kwargs: Any) -> SimpleNamespace:
-        captured.update(kwargs)
-        return SimpleNamespace(
-            id="user-3",
-            email="u3@example.com",
-            username="user3",
-            role=SimpleNamespace(value="user"),
-            is_active=True,
-            allowed_providers=[],
-            allowed_api_formats=[],
-            allowed_models=[],
-        )
+    def _create_user_sync(request: Any, role: Any) -> tuple[dict[str, Any], dict[str, Any]]:
+        captured["allowed_providers"] = request.allowed_providers
+        captured["allowed_api_formats"] = request.allowed_api_formats
+        captured["allowed_models"] = request.allowed_models
+        captured["role"] = role
+        return {"id": "user-3"}, {}
 
-    monkeypatch.setattr("src.api.admin.users.routes.UserService.create_user", _create_user)
-    monkeypatch.setattr(
-        "src.api.admin.users.routes._serialize_user",
-        lambda _db, user: {"id": user.id},
-    )
+    monkeypatch.setattr("src.api.admin.users.routes._create_user_sync", _create_user_sync)
 
     context = SimpleNamespace(
         db=db,