feat: add global RUST_ACCEL toggle to enable/disable rust acceleration at runtime

- Introduce RUST_ACCEL config to control all rust fast paths
- Fallback to Python implementations when disabled, preserving filter semantics
- Expose rust acceleration status in system info API
- Update CLI docs to reflect new toggle
- Add tests for runtime switch and fallback behavior

tests/test_rust_accel_toggle.py

@@ -0,0 +1,45 @@
+from app.core.config import settings
+from app.utils import rust_accel
+
+
+class _DummyRustExtension:
+    """
+    测试用 Rust 扩展替身，用来验证总开关会阻止扩展调用。
+    """
+
+    @staticmethod
+    def is_available() -> bool:
+        """
+        模拟 Rust 扩展基础能力可用。
+        """
+        return True
+
+    @staticmethod
+    def parse_filter_rule_fast(_expression: str) -> list:
+        """
+        如果总开关关闭后仍调用扩展，测试应立即失败。
+        """
+        raise AssertionError("Rust extension should not be called")
+
+
+def test_rust_accel_runtime_switch_disables_fast_paths(monkeypatch):
+    """
+    RUST_ACCEL 关闭时，即便扩展可用也应回退到 Python 路径。
+    """
+    monkeypatch.setattr(settings, "RUST_ACCEL", False)
+    monkeypatch.setattr(rust_accel, "_moviepilot_rust", _DummyRustExtension())
+
+    assert rust_accel.is_available()
+    assert not rust_accel.is_enabled()
+    assert rust_accel.parse_filter_rule("HDR") is None
+
+
+def test_rust_accel_status_reports_enabled_state(monkeypatch):
+    """
+    状态接口应同时体现扩展可用性和配置开关后的实际启用状态。
+    """
+    monkeypatch.setattr(settings, "RUST_ACCEL", True)
+    monkeypatch.setattr(rust_accel, "_moviepilot_rust", _DummyRustExtension())
+
+    assert rust_accel.status()["available"] is True
+    assert rust_accel.status()["enabled"] is True