feat: jc 引擎主线搬入 app/engine（315 测试基线全绿：320 去掉 admin 5 用例）

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

tests/test_rate_limit.py

@@ -0,0 +1,21 @@
+from app.engine.services.rate_limit import check, _reset_for_test
+
+
+def test_allows_under_cap():
+    _reset_for_test()
+    for _ in range(30):
+        assert check("1.2.3.4", "dev1") is True
+
+
+def test_blocks_over_cap():
+    _reset_for_test()
+    for _ in range(30):
+        assert check("1.2.3.4", "dev1") is True
+    assert check("1.2.3.4", "dev1") is False
+
+
+def test_isolated_buckets():
+    _reset_for_test()
+    for _ in range(30):
+        check("1.2.3.4", "dev1")
+    assert check("5.6.7.8", "dev2") is True  # different ip+device unaffected