fix(download): align location-step root selection with backend diffusion routing

The download modal's location step decided between checkpoint and unet
roots using only the CivitAI file-type signal, while the backend also
falls back to DIFFUSION_MODEL_BASE_MODELS. Models like Anima (file type
"Model") were offered checkpoint roots in the UI even though
use_default_paths would route them to the unet root.

- Extract the two-tier decision into py/services/download_routing.py and
  reuse it in DownloadManager._execute_download
- Add POST /api/lm/download/routing so the UI asks the backend for the
  routing decision; fall back to the local file-type check on failure
- ModelVersionsTab: search both checkpoint and unet roots when resolving
  an existing version's download path

py/services/download_routing.py

@@ -0,0 +1,53 @@
+"""Shared download routing logic.
+
+Decides whether a download initiated from the checkpoint library should be
+routed to the unet/diffusion-model roots instead of the checkpoint roots.
+Used by both the download manager (at download time) and the download
+routing HTTP endpoint (when the user picks a location in the UI), so the
+two can never disagree.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Iterable
+
+from ..utils.constants import DIFFUSION_MODEL_BASE_MODELS
+
+logger = logging.getLogger(__name__)
+
+# File types reported by the CivitAI API that indicate a raw diffusion
+# model (loaded via UNETLoader in ComfyUI) rather than a full checkpoint.
+DIFFUSION_FILE_TYPES = frozenset({"UNet", "Diffusion Model"})
+
+
+def is_diffusion_model_download(
+    model_type: str,
+    file_types: Iterable[str] = (),
+    base_model: str = "",
+) -> bool:
+    """Return True when a download should be routed to the unet roots.
+
+    Only applies to downloads initiated from the checkpoint library.
+    Priority: (1) any file has type "UNet" or "Diffusion Model" (the more
+    direct signal from CivitAI), (2) baseModel is a known diffusion model.
+    """
+    if model_type != "checkpoint":
+        return False
+
+    for file_type in file_types:
+        if file_type in DIFFUSION_FILE_TYPES:
+            logger.info(
+                "File type '%s' detected, routing checkpoint to unet folder",
+                file_type,
+            )
+            return True
+
+    if base_model in DIFFUSION_MODEL_BASE_MODELS:
+        logger.info(
+            "baseModel '%s' is a known diffusion model, routing to unet folder",
+            base_model,
+        )
+        return True
+
+    return False