perf: slim daw_engine bundle 409MB->170MB + fix engine khong chay tren Windows

- Thay librosa bang app/core/audio_features.py (numpy/scipy/soundfile):
  load, beat_track, frames_to_time, spectral_centroid, rms, zero_crossing_rate,
  time_stretch, pitch_shift, chroma_stft. A/B ngang librosa (BPM <1% sai lech,
  pitch_shift chuan toi Hz). Loai bo llvmlite 171MB + scikit-learn + numba.
- Task layer 2 che do (app/tasks/worker.py): server giu celery; desktop slim
  chay in-process thread + registry, giu nguyen API contract (.delay/.id/status
  /tasks/{id}) nen frontend khong doi.
- engine.spec: excludes librosa/numba/llvmlite/sklearn/celery/redis/kombu/
  billiard/amqp/msgpack/yaml/PIL/cairosvg/zstandard/...; scan scipy gioi han
  scipy.signal; giu click (uvicorn.main import click).
- render_engine: scipy.signal thanh lazy import (giam cold start).
- tauri.conf.json: targets [nsis, msi] - NSIS tro lai (bundle nho) de
  hooks.nsh cai VC++ Redistributable - sua bug daw_engine.exe khong chay
  tren Windows (truoc day MSI-only khong chay hooks).
- build_linux.sh / build_macos.sh: build 1 lenh moi OS.
- Doc: DESKTOP_INSTALL_PLAN.md muc 5.1.
- Verify: 86 tests pass, engine dong goi upload/analyze/waveform/export OK.

app/api/v1/tasks.py

@@ -1,12 +1,19 @@
+import os
 from fastapi import APIRouter
-from celery.result import AsyncResult
-from app.tasks.worker import celery_app
 
 router = APIRouter()
 
+# Task status endpoint dung chung cho ca 2 che do:
+# - Server/Docker: celery (AsyncResult, broker Redis).
+# - Desktop slim (PyInstaller khong bundle celery): in-process registry
+#   (app/tasks/worker._SimpleAsyncResult) — API contract giong het nhau.
+
+
 @router.get("/tasks/{task_id}")
 async def get_task_status(task_id: str):
-    res = AsyncResult(task_id, app=celery_app)
+    from app.tasks.worker import get_task_result
+
+    res = get_task_result(task_id)
     response_data = {
         "task_id": task_id,
         "status": res.status,