Add ComfyUI models documentation and download script

- Complete MODELS.md with detailed model descriptions
- Interactive download script for 5+ specialized SDXL models
- Updated README with model selection guide
- Fix docker-compose volumes path (/root/ComfyUI)
- API workflows for OpenWebUI integration

Models documented:
- RealVisXL (portraits)
- Juggernaut XL (versatile)
- DreamShaper (art/fantasy)
- SDXL Lightning (fast)
- Proteus (realism)

comfyui/workflows/openwebui_api.json

@@ -0,0 +1,59 @@
+{
+  "3": {
+    "inputs": {
+      "seed": 0,
+      "steps": 30,
+      "cfg": 8.0,
+      "sampler_name": "euler",
+      "scheduler": "normal",
+      "denoise": 1.0,
+      "model": ["4", 0],
+      "positive": ["6", 0],
+      "negative": ["7", 0],
+      "latent_image": ["5", 0]
+    },
+    "class_type": "KSampler"
+  },
+  "4": {
+    "inputs": {
+      "ckpt_name": "sd_xl_base_1.0.safetensors"
+    },
+    "class_type": "CheckpointLoaderSimple"
+  },
+  "5": {
+    "inputs": {
+      "width": 1024,
+      "height": 1024,
+      "batch_size": 1
+    },
+    "class_type": "EmptyLatentImage"
+  },
+  "6": {
+    "inputs": {
+      "text": "",
+      "clip": ["4", 1]
+    },
+    "class_type": "CLIPTextEncode"
+  },
+  "7": {
+    "inputs": {
+      "text": "text, watermark, blurry, low quality",
+      "clip": ["4", 1]
+    },
+    "class_type": "CLIPTextEncode"
+  },
+  "8": {
+    "inputs": {
+      "samples": ["3", 0],
+      "vae": ["4", 2]
+    },
+    "class_type": "VAEDecode"
+  },
+  "9": {
+    "inputs": {
+      "filename_prefix": "ComfyUI",
+      "images": ["8", 0]
+    },
+    "class_type": "SaveImage"
+  }
+}