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>

.env.example

@@ -1,11 +1,13 @@
 # SparkyUI - ComfyUI for DGX Spark (Blackwell GB10)
 # Copy this to .env and customize paths as needed
 
-# Base path where your existing ComfyUI installation lives (for models)
-COMFYUI_HOST_PATH=/path/to/your/ComfyUI
+# Base path that holds the models/ directory (defaults to the project root).
+# The Model Manager downloads into <COMFYUI_HOST_PATH>/models and ComfyUI reads it.
+COMFYUI_HOST_PATH=.
 
-# Base path for SparkyUI data (custom_nodes, outputs, inputs, etc.)
-SPARKYUI_DATA_PATH=/path/to/SparkyUI
+# Base path for SparkyUI data (custom_nodes, outputs, inputs, manager DB, etc.)
+# Defaults to the project root.
+SPARKYUI_DATA_PATH=.
 
 # ComfyUI settings
 COMFYUI_PORT=8188
@@ -25,3 +27,7 @@ SAGEATTN_REF=main
 # Access at http://<host>:3000
 COMFYUIMINI_PORT=3000
 COMFYUIMINI_REF=main
+
+# Model Manager - StabilityMatrix-style model download/management UI
+# Access at http://<host>:8189
+MODEL_MANAGER_PORT=8189