fix(downloads): route unknown checkpoint baseModels to diffusion models by default

CivitAI labels new DiT architectures (MiniMax H3, future Flux/Wan/Qwen
variants) as model.type "Checkpoint" with plain "Model" file entries,
so the DIFFUSION_MODEL_BASE_MODELS allowlist could never keep up and
such downloads were mis-routed to the checkpoint roots (e.g. model
2877206 / version 3374439). The set of true full-checkpoint families is
closed, so the baseModel fallback is inverted:

1. file type UNet/Diffusion Model -> unet (unchanged)
2. baseModel in DIFFUSION_MODEL_BASE_MODELS (now incl. MiniMax H3) -> unet
3. baseModel in new CHECKPOINT_BASE_MODELS (SD 1.x/2.x/3.x, SDXL, Pony,
   Illustrious, NoobAI) -> checkpoint
4. unknown/empty baseModel -> new unknown_base_model_routing setting,
   defaulting to diffusion models

The setting is exposed under Settings > Downloads, validated in
SettingsManager, and threaded into both the download manager and the
download routing endpoint so they keep agreeing.

tests/routes/test_download_routing_handlers.py

@@ -73,6 +73,42 @@ async def test_regular_checkpoint_stays_on_checkpoint_root():
     assert payload["root_kind"] == "checkpoint"
 
 
+@pytest.mark.asyncio
+async def test_unknown_base_model_routes_to_unet_by_default():
+    handler = DownloadRoutingHandler()
+    response = await handler.get_download_routing(
+        FakeRequest(
+            {
+                "model_type": "checkpoint",
+                "base_model": "Brand New Arch",
+                "file_types": ["Model"],
+            }
+        )
+    )
+    payload = json.loads(response.text)
+    assert payload["is_diffusion_model"] is True
+    assert payload["root_kind"] == "unet"
+
+
+@pytest.mark.asyncio
+async def test_unknown_base_model_honors_checkpoint_routing_setting():
+    get_settings_manager().settings["unknown_base_model_routing"] = "checkpoint"
+
+    handler = DownloadRoutingHandler()
+    response = await handler.get_download_routing(
+        FakeRequest(
+            {
+                "model_type": "checkpoint",
+                "base_model": "Brand New Arch",
+                "file_types": ["Model"],
+            }
+        )
+    )
+    payload = json.loads(response.text)
+    assert payload["is_diffusion_model"] is False
+    assert payload["root_kind"] == "checkpoint"
+
+
 @pytest.mark.asyncio
 async def test_lora_is_never_diffusion():
     handler = DownloadRoutingHandler()