feat: add CosyVoice2/3 TTS engine with instruct and voice cloning

Integrate Alibaba's CosyVoice2-0.5B and Fun-CosyVoice3-0.5B as a new
TTS engine supporting 9 languages, zero-shot voice cloning, and instruct
control (emotions, speed, volume, dialects).

The CosyVoice source is cloned at setup time into backend/vendors/ since
no PyPI package exists. A modelscope→HuggingFace shim redirects model
downloads to the public HF repos, and a lightweight pylogger shim avoids
pulling in pytorch-lightning as a transitive dependency.

Backend: cosyvoice_backend.py, __init__.py registry, models.py regex
Frontend: engine selector, language map, Zod schema, model descriptions
Infra: requirements.txt, justfile, release.yml, Dockerfile, PyInstaller

justfile

@@ -48,6 +48,12 @@ setup-python:
     {{ pip }} install --no-deps chatterbox-tts
     # HumeAI TADA pins torch>=2.7,<2.8 which conflicts with our torch>=2.1
     {{ pip }} install --no-deps hume-tada
+    # CosyVoice: clone source into backend/vendors/ (no PyPI package exists)
+    if [ ! -d "{{ backend_dir }}/vendors/CosyVoice" ]; then
+        echo "Cloning CosyVoice source..."
+        mkdir -p {{ backend_dir }}/vendors
+        git clone --recursive --depth 1 https://github.com/FunAudioLLM/CosyVoice.git {{ backend_dir }}/vendors/CosyVoice
+    fi
     # Apple Silicon: install MLX backend
     if [ "$(uname -m)" = "arm64" ] && [ "$(uname)" = "Darwin" ]; then
         echo "Detected Apple Silicon — installing MLX dependencies..."
@@ -77,6 +83,11 @@ setup-python:
     & "{{ pip }}" install -r {{ backend_dir }}/requirements.txt
     & "{{ pip }}" install --no-deps chatterbox-tts
     & "{{ pip }}" install --no-deps hume-tada
+    if (-not (Test-Path "{{ backend_dir }}/vendors/CosyVoice")) { \
+        Write-Host "Cloning CosyVoice source..."; \
+        New-Item -ItemType Directory -Force -Path "{{ backend_dir }}/vendors" | Out-Null; \
+        git clone --recursive --depth 1 https://github.com/FunAudioLLM/CosyVoice.git "{{ backend_dir }}/vendors/CosyVoice"; \
+    }
     & "{{ pip }}" install git+https://github.com/QwenLM/Qwen3-TTS.git
     & "{{ pip }}" install pyinstaller ruff pytest pytest-asyncio -q
     Write-Host "Python environment ready."