feat(download): auto-route diffusion models to unet folder based on baseModel, see #770

CivitAI does not distinguish between checkpoint and diffusion model types -
both are labeled as "checkpoint". For certain base model types like
"ZImageTurbo", all models are actually diffusion models and should be
saved to the unet/diffusion model folder instead of the checkpoint folder.

- Add DIFFUSION_MODEL_BASE_MODELS constant for known diffusion model types
- Add default_unet_root setting with auto-set logic
- Route downloads to unet folder when baseModel matches known diffusion types

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

py/utils/constants.py

@@ -75,3 +75,9 @@ DEFAULT_PRIORITY_TAG_CONFIG = {
     'checkpoint': ', '.join(CIVITAI_MODEL_TAGS),
     'embedding': ', '.join(CIVITAI_MODEL_TAGS),
 }
+
+# baseModel values from CivitAI that should be treated as diffusion models (unet)
+# These model types are incorrectly labeled as "checkpoint" by CivitAI but are actually diffusion models
+DIFFUSION_MODEL_BASE_MODELS = frozenset([
+    "ZImageTurbo",
+])