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

static/js/components/shared/ModelVersionsTab.js

@@ -1390,8 +1390,22 @@ export function initVersionsTab({
 
         try {
             const client = ensureClient();
-            const rootsData = await client.fetchModelRoots();
-            const roots = rootsData?.roots;
+            // On the checkpoints page a diffusion model lives under the unet
+            // roots, so both root sets are needed to locate the current file.
+            let roots;
+            if (modelType === 'checkpoints') {
+                const [checkpointRoots, unetRoots] = await Promise.all([
+                    client.fetchModelRoots(),
+                    client.fetchModelRoots('diffusion_model'),
+                ]);
+                roots = [
+                    ...(checkpointRoots?.roots || []),
+                    ...(unetRoots?.roots || []),
+                ];
+            } else {
+                const rootsData = await client.fetchModelRoots();
+                roots = rootsData?.roots;
+            }
             if (!Array.isArray(roots) || roots.length === 0) {
                 return null;
             }