added Stable Diffusion 3 support alongside ComfyUI configuration

this commit adds four environment variables:

- COMFYUI_CFG_SCALE
- COMFYUI_SAMPLER
- COMFYUI_SCHEDULER
- COMFYUI_SD3 (merely setting this at all will enable SD3 mode)

backend/config.py

@@ -1000,6 +1000,30 @@ COMFYUI_BASE_URL = PersistentConfig(
     os.getenv("COMFYUI_BASE_URL", ""),
 )
 
+COMFYUI_CFG_SCALE = PersistentConfig(
+    "COMFYUI_CFG_SCALE",
+    "image_generation.comfyui.cfg_scale",
+    os.getenv("COMFYUI_CFG_SCALE", ""),
+)
+
+COMFYUI_SAMPLER = PersistentConfig(
+    "COMFYUI_SAMPLER",
+    "image_generation.comfyui.sampler",
+    os.getenv("COMFYUI_SAMPLER", ""),
+)
+
+COMFYUI_SCHEDULER = PersistentConfig(
+    "COMFYUI_SCHEDULER",
+    "image_generation.comfyui.scheduler",
+    os.getenv("COMFYUI_SCHEDULER", ""),
+)
+
+COMFYUI_SD3 = PersistentConfig(
+    "COMFYUI_SD3",
+    "image_generation.comfyui.sd3",
+    os.environ.get("COMFYUI_SD3", "").lower() == "true",
+)
+
 IMAGES_OPENAI_API_BASE_URL = PersistentConfig(
     "IMAGES_OPENAI_API_BASE_URL",
     "image_generation.openai.api_base_url",