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>

.gitignore

@@ -54,6 +54,13 @@ input/*
 workflows/*
 !workflows/.gitkeep
 
+# Downloaded models - ignore contents but keep the directory
+models/*
+!models/.gitkeep
+
+# Model Manager persistent data (SQLite DB, stored API keys)
+sparkyui-data/
+
 # Wheels directory - for prebuilt ARM64/sm_121 binaries
 # Ignore contents except .gitkeep (add wheels explicitly if needed)
 wheels/*