V45: D17 seamless loop wrap + D18 stop solo note-off (Qin) + D18b/c guard + engine VST load fixes

- D17: wrap_info/wrap-skip, fx wrap xfade, playhead-derived scheduling
- D18: solo stop -> NOTE_OFF all sounding timeline pitches + CC123/CC120
- D18b: track audible gain guard, tracksRef fallback
- D18c: timelinePitchesRef per-track Set, kill at applyAllTrackMuteSolo
- bridge ring _MAX 16, index v=202608290928
- plugins.py/vst_engine.py/fx_realtime.py/JuceFxEngine.cpp: VSTi load + Qin fixes (uncommitted 12:18)

app/api/v1/plugins.py

@@ -1,13 +1,10 @@
 import os, sys, uuid, json, tempfile, subprocess, time as _time, threading, asyncio, logging
-import numpy as np
-import soundfile as sf
 from fastapi import APIRouter, HTTPException, Depends, UploadFile, File, BackgroundTasks, Query, WebSocket, WebSocketDisconnect
 from fastapi.responses import FileResponse
 from pydantic import BaseModel
 from typing import Optional, Any
 from app.config import settings
 from app.core.vst_engine import PluginManager, HAS_PYFLUIDSYNTH, render_soundfont_midi_to_audio, RENDER_LOCK, _render_locked
-from app.core.render_engine import PythonRenderEngine, _find_sf2_path, _find_default_sf2
 from app.core.soundfont_inspector import SoundFontInspector
 from app.core.soundfont_converter import SoundFontConverter
 from app.core.soundfont_scanner import SoundFontAutoScanner
@@ -570,6 +567,7 @@ async def delete_soundfont(sf_id: str, current_user: dict = Depends(get_current_
 
 @router.get("/soundfonts/download/{sf_id}")
 async def download_soundfont_asset(sf_id: str):
+    from app.core.render_engine import _find_sf2_path, _find_default_sf2
     clean_id = sf_id.replace("sf_", "") if sf_id.startswith("sf_") else sf_id
     # Cũng tìm trong static/soundfonts (font bundled theo deployment) — trước
     # đây chỉ UPLOAD + SYSTEM → font bundled 404 → incognito (IndexedDB rỗng)
@@ -963,6 +961,7 @@ class FxRenderRequest(BaseModel):
 @router.post("/fx-render")
 async def fx_render(req: FxRenderRequest, current_user: dict = Depends(get_current_user)):
     """Áp fx_chain lên WAV đầu vào → WAV mới trong PROCESSED_DIR."""
+    import soundfile as sf
     enforce_password_changed(current_user)
     src = (req.input_path or "").strip()
     if not src:
@@ -1233,6 +1232,8 @@ async def soundfont_render(req: SoundfontRenderRequest, current_user: dict = Dep
     Khác FluidSynthWASM (docker): âm render server-side bằng pyfluidsynth, trả
     WAV để client play. Dùng cho: preview/keyboard, play MIDI item khi chạy
     transport ở môi trường standalone."""
+    from app.core.render_engine import _find_sf2_path, _find_default_sf2
+    import soundfile as sf
     enforce_password_changed(current_user)
     if not req.notes:
         raise HTTPException(status_code=400, detail="Chưa có nốt nhạc để render")
@@ -1288,6 +1289,9 @@ def render_project(
     current_user: dict = Depends(get_current_user)
 ):
     enforce_password_changed(current_user)
+    from app.core.render_engine import PythonRenderEngine
+    import soundfile as sf
+    import numpy as np
     engine = PythonRenderEngine()
     # Prevent path traversal: strip directory components; force .wav/.ogg.
     safe_name = os.path.basename((req.output_filename or "render_output.wav").replace("\\", "/"))