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>

README.md

@@ -26,6 +26,7 @@ Standard ComfyUI containers and PyTorch wheels don't support sm_121. SparkyUI so
 - **ComfyUI** (latest master branch)
 - **ComfyUI-Manager** - auto-installed on first run for easy custom node management
 - **ComfyUIMini** - mobile-friendly web UI for phones/tablets (separate container)
+- **Model Manager** - StabilityMatrix-style UI to download/manage models (separate container)
 - **SageAttention** - compiled natively for sm_121 (Blackwell tensor cores)
 - **PyTorch 2.9.1+cu130** - ARM64 wheels with CUDA 13.0 support
 
@@ -77,6 +78,7 @@ docker compose logs -f
 **Access:**
 - **ComfyUI (Desktop):** http://localhost:8188
 - **ComfyUIMini (Mobile):** http://localhost:3000
+- **Model Manager:** http://localhost:8189
 
 ## Requirements
 
@@ -91,17 +93,28 @@ docker compose logs -f
 Copy `.env.example` to `.env` and edit:
 
 ```bash
-# Path to your existing ComfyUI models (mounted read-only)
-COMFYUI_HOST_PATH=/path/to/your/ComfyUI
+# Base path holding the models/ directory (defaults to the project root).
+# The Model Manager downloads into <COMFYUI_HOST_PATH>/models; ComfyUI reads it.
+COMFYUI_HOST_PATH=.
 
-# Path for SparkyUI data (custom_nodes, outputs, inputs)
-SPARKYUI_DATA_PATH=/path/to/SparkyUI
+# Path for SparkyUI data (custom_nodes, outputs, inputs, manager DB).
+# Defaults to the project root.
+SPARKYUI_DATA_PATH=.
+
+# Ports
+COMFYUI_PORT=8188
+COMFYUIMINI_PORT=3000
+MODEL_MANAGER_PORT=8189
 
 # Optional: pin to specific versions
 COMFYUI_REF=master
 SAGEATTN_REF=main
 ```
 
+Both paths default to the project root, so out of the box models are stored in
+`./models` and the Model Manager's database in `./sparkyui-data`. Point
+`COMFYUI_HOST_PATH` at an existing ComfyUI install if you'd rather reuse its models.
+
 ## Architecture
 
 ```
@@ -214,6 +227,36 @@ docker compose build comfyuimini
 docker compose up -d comfyuimini
 ```
 
+## Model Manager
+
+SparkyUI includes a **StabilityMatrix-style Model Manager** - a lightweight FastAPI web app
+(separate container) for downloading and managing models without touching the command line.
+
+**Access:** `http://<your-dgx-ip>:8189`
+
+**Features:**
+- **Installed Models** - browse what's on disk, grouped by type, with size and delete actions
+- **Add / Download** - paste a download URL and pick a type; live progress bars
+  - **Direct URLs** - any direct download link
+  - **CivitAI** - paste a model page link (`civitai.com/models/...`) or an
+    `api/download/models/...` link; the type and filename are auto-detected
+  - **HuggingFace** - paste a `resolve` URL (works with gated repos via your token)
+- **Settings** - store your **CivitAI API key** and **HuggingFace token** persistently
+  (saved to a SQLite DB under `./sparkyui-data`, never committed to git)
+
+**How it works:**
+- Runs as a FastAPI server in its own container (`python:3.12-slim`)
+- Downloads land in the shared `models/` folder, sorted into ComfyUI's standard sub-folders
+  by type (`checkpoints/`, `loras/`, `vae/`, `controlnet/`, `upscale_models/`, …) - these are
+  created automatically on first start
+- ComfyUI mounts the same `models/` folder read-only, so new downloads appear in its loaders
+
+**Build only the Model Manager** (if the rest is already built):
+```bash
+docker compose build model-manager
+docker compose up -d model-manager
+```
+
 ## SageAttention Notes
 
 SageAttention PR #297 added sm_121 support but was merged then reverted due to stability issues. Our approach: