fix(standalone): bundle pedalboard.midi_utils + pyfluidsynth dll path

- engine.spec hiddenimports: pedalboard.midi_utils (C++ ExternalPlugin.h
  import dong, PyInstaller khong phat hien duoc -> midi-render 500)
- engine.spec hiddenimports: fluidsynth (pyfluidsynth cffi) cho
  /soundfont-render native
- vst_engine.py: _ensure_fluidsynth_dll_on_path() - frozen app them
  exe_dir vao PATH de ctypes.util.find_library tim thay
  libfluidsynth-3.dll dat ke exe

engine.spec

@@ -90,8 +90,16 @@ _hidden = [
     'uvicorn.protocols.websockets',
     'uvicorn.protocols.websockets.auto',
     'pedalboard',
+    # C++ ExternalPlugin.h import pedalboard.midi_utils khi tao VST3Plugin
+    # (py::module_::import) - PyInstaller khong thay duoc import tu C++, phai
+    # khai bao hiddenimport thu cong (thieu -> "No module named
+    # 'pedalboard.midi_utils'" khi /midi-render).
+    'pedalboard.midi_utils',
     'soundfile',
     'sf2utils',
+    # pyfluidsynth (fluidsynth.py cffi) - render soundfont native cho
+    # /soundfont-render (browser client). Can libfluidsynth-3.dll ke exe.
+    'fluidsynth',
     'mido.backends.rtmidi',
 ] + _scipy_signal_hidden