fix(fx-list): MASTERING PANEL khong hien thi VST FX da scan trong Plugin Manager (cache _fx_scan_all ket [] vinh vien)

- vst_engine: list_scan_all cache co TTL 300s (FX_SCAN_TTL) + chi cache khi
  find_fx_bridge_exe() co exe (thieu bridge -> tra [] NHUNG khong cache de
  lan sau thu lai) — truoc day ket [] vi mui gio, MASTERING PANEL rong.
- vst_engine: them invalidate_fx_scan() clear cache scan VST.
- plugins.py: POST /scan (Plugin Manager Scan) goi _invalidate_fx_scan_cache()
  -> MASTERING PANEL lan mo ke tiep quet lai voi dirs moi; response them
  fx_cache_invalidated.
- app.jsx: MasteringModal/FXRackModal load listFx() lai moi lan mo modal
  (deps [isOpen]/[track.id]) + loading state; VstPluginPicker nhan prop
  loading -> hien 'Dang quet VST FX...' thay vi 'Khong co VST FX'.
- precompiled rebuild (LF) + index.html ?v=202608220330; dist sync.
- tests: +4 test cache TTL/invalidate/no-bridge (TestFxScanCache).

tests/test_vst_engine.py

@@ -54,6 +54,72 @@ class TestPluginManager:
         assert PluginManager.validate_sf2_header(b'RIFF') is False
 
 
+class TestFxScanCache:
+    """list_scan_all cache TTL + invalidate — fix MASTERING PANEL không hiển thị
+    VST FX đã scan trong Plugin Manager (cache kẹt [] vĩnh viễn)."""
+
+    SCAN_JSON = json.dumps({"plugins": [
+        {"path": "C:\\VST3\\Ozone Pro.vst3", "name": "Ozone Pro",
+         "type": "VST3", "is_fx": True, "is_instrument": False},
+    ]})
+
+    def _pm(self, tmp_path):
+        d = str(tmp_path)
+        return PluginManager(vst_dir=d, sf_dir=d, extra_vst_dirs=[])
+
+    def test_scan_caches_with_bridge(self, tmp_path):
+        pm = self._pm(tmp_path)
+        with patch("app.core.native_render.find_fx_bridge_exe", return_value="fx_bridge.exe"), \
+             patch("subprocess.run", return_value=MagicMock(
+                 returncode=0, stdout=self.SCAN_JSON, stderr="")) as mrun:
+            r1 = pm.list_scan_all()
+            assert len(r1) == 1 and r1[0]["name"] == "Ozone Pro"
+            assert pm._fx_scan_all is not None and len(pm._fx_scan_all) == 1
+            # Lần 2 trong TTL → KHÔNG chạy lại bridge
+            mrun.reset_mock()
+            r2 = pm.list_scan_all()
+            assert r2 == r1
+            mrun.assert_not_called()
+
+    def test_scan_no_bridge_does_not_cache_empty(self, tmp_path):
+        """Bridge chưa tồn tại → trả [] nhưng KHÔNG cache → lần sau thử lại
+        (trước đây cache [] vĩnh viễn → MASTERING PANEL trống)."""
+        pm = self._pm(tmp_path)
+        with patch("app.core.native_render.find_fx_bridge_exe", return_value=""):
+            assert pm.list_scan_all() == []
+        assert pm._fx_scan_all is None  # KHÔNG cache rỗng
+        # Có bridge → scan thật chạy
+        with patch("app.core.native_render.find_fx_bridge_exe", return_value="fx_bridge.exe"), \
+             patch("subprocess.run", return_value=MagicMock(
+                 returncode=0, stdout=self.SCAN_JSON, stderr="")):
+            r = pm.list_scan_all()
+            assert len(r) == 1
+
+    def test_scan_ttl_expiry_rescans(self, tmp_path):
+        pm = self._pm(tmp_path)
+        with patch("app.core.native_render.find_fx_bridge_exe", return_value="fx_bridge.exe"), \
+             patch("subprocess.run", return_value=MagicMock(
+                 returncode=0, stdout=self.SCAN_JSON, stderr="")) as mrun:
+            pm.list_scan_all()
+            mrun.assert_called_once()
+            # Giả TTL đã hết hạn → gọi lại phải scan
+            pm._fx_scan_all_ts -= (pm.FX_SCAN_TTL + 1)
+            pm.list_scan_all()
+            assert mrun.call_count == 2
+
+    def test_invalidate_fx_scan(self, tmp_path):
+        pm = self._pm(tmp_path)
+        with patch("app.core.native_render.find_fx_bridge_exe", return_value="fx_bridge.exe"), \
+             patch("subprocess.run", return_value=MagicMock(
+                 returncode=0, stdout=self.SCAN_JSON, stderr="")):
+            pm.list_scan_all()
+            assert pm._fx_scan_all is not None
+        pm.invalidate_fx_scan()
+        assert pm._fx_scan_all is None
+        assert pm._fx_scan_all_ts == 0.0
+        assert pm._fx_scan_cache is None
+
+
 class TestMidiNoteToFreq:
     def test_a4_440(self):
         assert midi_note_to_freq(69) == 440.0