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/routes/handlers/download_routing_handlers.py

@@ -0,0 +1,59 @@
+"""HTTP handler for download target routing decisions."""
+
+from __future__ import annotations
+
+import json
+import logging
+
+from aiohttp import web
+
+from ...services.download_routing import is_diffusion_model_download
+
+logger = logging.getLogger(__name__)
+
+
+class DownloadRoutingHandler:
+    """Expose the download-time checkpoint/diffusion-model routing decision.
+
+    The web UI calls this when the user reaches the download location step
+    so the root dropdown offers the same root set (checkpoint vs unet) that
+    the download manager would pick for ``use_default_paths``.
+    """
+
+    async def get_download_routing(self, request: web.Request) -> web.Response:
+        try:
+            payload = await request.json()
+        except json.JSONDecodeError:
+            return web.json_response(
+                {"success": False, "error": "Invalid JSON payload"}, status=400
+            )
+
+        model_type = payload.get("model_type", "")
+        base_model = payload.get("base_model") or ""
+        file_types = payload.get("file_types") or []
+
+        if not isinstance(model_type, str) or not model_type:
+            return web.json_response(
+                {"success": False, "error": "model_type is required"}, status=400
+            )
+        if not isinstance(base_model, str) or not isinstance(file_types, list):
+            return web.json_response(
+                {
+                    "success": False,
+                    "error": "base_model must be a string and file_types a list",
+                },
+                status=400,
+            )
+
+        is_diffusion = is_diffusion_model_download(
+            model_type,
+            file_types=(str(t) for t in file_types),
+            base_model=base_model,
+        )
+        return web.json_response(
+            {
+                "success": True,
+                "is_diffusion_model": is_diffusion,
+                "root_kind": "unet" if is_diffusion else model_type,
+            }
+        )

py/routes/handlers/misc_handlers.py

@@ -56,6 +56,7 @@ from ...utils.constants import (
 )
 from .hf_handlers import HfHandler
 from .agent_handlers import AgentHandler
+from .download_routing_handlers import DownloadRoutingHandler
 from .model_handlers import ModelCivitaiHandler
 from ...utils.civitai_utils import rewrite_preview_url
 from ...utils.example_images_paths import (
@@ -3884,6 +3885,7 @@ class MiscHandlerSet:
         base_model: BaseModelHandlerSet,
         hf_handler: Any = None,
         agent_handler: Any = None,
+        download_routing: Any = None,
     ) -> None:
         self.health = health
         self.settings = settings
@@ -3904,6 +3906,7 @@ class MiscHandlerSet:
         self.base_model = base_model
         self.hf_handler = hf_handler
         self.agent_handler = agent_handler
+        self.download_routing = download_routing
 
     def to_route_mapping(
         self,
@@ -3962,6 +3965,8 @@ class MiscHandlerSet:
             "get_agent_skills": self.agent_handler.get_agent_skills,
             "execute_agent_skill": self.agent_handler.execute_agent_skill,
             "cancel_agent_skill": self.agent_handler.cancel_agent_skill,
+            # Download routing handler
+            "get_download_routing": self.download_routing.get_download_routing,
             # Base model handlers
             "get_base_models": self.base_model.get_base_models,
             "refresh_base_models": self.base_model.refresh_base_models,