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/core/sub_tab_dsp.py

@@ -1,6 +1,6 @@
 import numpy as np
 import scipy.signal as signal
-import librosa
+from app.core.audio_features import time_stretch as _time_stretch, pitch_shift as _pitch_shift
 
 class SubTabDSPEngine:
     @staticmethod
@@ -12,7 +12,7 @@ class SubTabDSPEngine:
             return y
             
         if preserve_pitch:
-            return librosa.effects.time_stretch(y, rate=speed_ratio)
+            return _time_stretch(y, rate=speed_ratio)
         else:
             num_samples_new = int(len(y) / speed_ratio)
             return signal.resample(y, num_samples_new)
@@ -80,7 +80,7 @@ class SubTabDSPEngine:
         """
         if n_steps == 0:
             return y
-        return librosa.effects.pitch_shift(y, sr=sr, n_steps=n_steps)
+        return _pitch_shift(y, sr=sr, n_steps=n_steps)
 
     @staticmethod
     def merge_back_to_parent(