feat: add StabilityMatrix-style Model Manager service

New FastAPI container (port 8189) to download and manage models:
- Installed Models, Add/Download (CivitAI/HuggingFace/direct URL), Settings views
- Persistent SQLite storage for API keys and download history (./sparkyui-data)
- Downloads land in ./models, auto-sorted into ComfyUI's standard subfolders
- Default COMFYUI_HOST_PATH and SPARKYUI_DATA_PATH to the project root
- Wire docker-compose service, env defaults, gitignore, README docs

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model-manager/.dockerignore

@@ -0,0 +1,9 @@
+__pycache__/
+*.py[cod]
+*.egg-info/
+.venv/
+venv/
+.pytest_cache/
+*.db
+*.db-wal
+*.db-shm