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/render_engine.py

@@ -1,7 +1,8 @@
 import os, logging, math
 import numpy as np
 import soundfile as sf
-import scipy.signal as signal
+# scipy.signal import LAZY (chi dung trong ham) — giam thoi gian khoi dong
+# engine (khong nap scipy+OpenBLAS ~70MB luc boot)
 from app.config import settings
 from app.core.vst_engine import (
     render_midi_events_to_audio,
@@ -385,7 +386,8 @@ class PythonRenderEngine:
                         for ch in range(2):
                             ir = ir_l if ch == 0 else ir_r
                             # Convolve
-                            conv = signal.convolve(track_buffer[ch, :], ir, mode='full')[:total_samples]
+                            from scipy.signal import convolve
+                            conv = convolve(track_buffer[ch, :], ir, mode='full')[:total_samples]
                             wet[ch, :] = conv
                         track_buffer = dry + wet * 0.4
                     except Exception as e: