fix(downloads): close routing gaps and map CivitAI ModelType.UNet to the checkpoint branch

Cross-checked both baseModel lists against CivitAI's official
baseModelRecords (packages/civitai-shared src/basemodel.constants.ts):

- CHECKPOINT_BASE_MODELS gains SD 2.0/2.1 768, SD 2.1 Unclip, SDXL 0.9 /
  1.0 LCM / Turbo / Distilled, Playground v2 and Stable Cascade
  (unCLIP-style but CheckpointLoader-loaded).
- DIFFUSION_MODEL_BASE_MODELS gains SVD XT, LTXV 2.5, Flux 3 Video,
  Wan Image 2.7, Wan Video 2.7 / 3.0, HiDream-O1, Boogu and the Ming
  Image Design families. API-only (Kling/Sora/Veo/Imagen...), 3D and
  audio baseModels are intentionally skipped.
- Pony V7 exclusion now backed by live-API evidence (model 1901521 is
  AuraFlow-architecture shipping .gguf variants).

CivitAI has no model-level diffusion ModelType: DiT models are uploaded
as "Checkpoint" or "UNet", with only uploader-chosen file types to tell
them apart. model.type "unet" previously fell through type derivation
and failed with 'not supported for download'; it now goes through the
checkpoint branch in both the download manager and the download routing
endpoint, so the standard chain (file type -> baseModel lists -> unknown
default) applies.

tests/services/test_download_manager_basic.py

@@ -2200,6 +2200,54 @@ async def test_download_uses_raw_file_name_from_mini_endpoint(
     ]
 
 
+@pytest.mark.asyncio
+async def test_unet_model_type_download_lands_in_unet_roots(
+    monkeypatch, scanners, metadata_provider, tmp_path
+):
+    """CivitAI ModelType.UNet goes through the checkpoint branch: its
+    UNet-typed files route to the unet roots like any diffusion model."""
+    manager = DownloadManager()
+    unet_root = tmp_path / "unet"
+    get_settings_manager().settings["default_unet_root"] = str(unet_root)
+    metadata_provider.payload = {
+        "id": 42,
+        "model": {"type": "UNet", "tags": ["fantasy"]},
+        "baseModel": "MiniMax H3",
+        "creator": {"username": "Author"},
+        "files": [
+            {
+                "id": 1001,
+                "type": "UNet",
+                "primary": True,
+                "name": "minimax_h3.safetensors",
+                "downloadUrl": "https://example.invalid/file.safetensors",
+            }
+        ],
+    }
+
+    captured = {}
+
+    async def fake_execute_download(self, **kwargs):
+        captured["file_path"] = kwargs["metadata"].file_path
+        return {"success": True}
+
+    monkeypatch.setattr(
+        DownloadManager, "_execute_download", fake_execute_download, raising=False
+    )
+
+    result = await manager.download_from_civitai(
+        model_version_id=42,
+        save_dir=str(tmp_path),
+        use_default_paths=True,
+        progress_callback=None,
+        source=None,
+    )
+
+    assert result["success"] is True, result
+    assert captured["file_path"].startswith(str(unet_root))
+    assert captured["file_path"].endswith("minimax_h3.safetensors")
+
+
 @pytest.mark.asyncio
 async def test_download_falls_back_to_rest_name_when_mini_fails(
     monkeypatch, scanners, metadata_provider, tmp_path