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/services/test_download_routing.py

@@ -20,9 +20,47 @@ def test_base_model_fallback_routes_to_unet():
     )
 
 
-def test_regular_checkpoint_stays_on_checkpoint_roots():
+def test_minimax_h3_routes_to_unet():
+    """CivitAI model 2877206: type "Checkpoint", baseModel "MiniMax H3",
+    all file entries typed "Model"."""
+    assert is_diffusion_model_download(
+        "checkpoint", file_types=["Model"], base_model="MiniMax H3"
+    )
+
+
+@pytest.mark.parametrize("base_model", ["SDXL 1.0", "Illustrious", "SD 1.5", "SD 3.5 Large"])
+def test_known_checkpoint_base_models_stay_on_checkpoint_roots(base_model):
+    """CHECKPOINT_BASE_MODELS members never route to unet, even when the
+    unknown-base-model default is diffusion."""
     assert not is_diffusion_model_download(
-        "checkpoint", file_types=["Model"], base_model="SDXL 1.0"
+        "checkpoint", file_types=["Model"], base_model=base_model
+    )
+
+
+def test_unknown_base_model_defaults_to_unet():
+    """New DiT families appear faster than the allowlist can track them, so
+    unknown baseModels route to unet by default."""
+    assert is_diffusion_model_download(
+        "checkpoint", file_types=["Model"], base_model="Brand New Arch"
+    )
+
+
+def test_unknown_base_model_honors_checkpoint_setting():
+    assert not is_diffusion_model_download(
+        "checkpoint",
+        file_types=["Model"],
+        base_model="Brand New Arch",
+        unknown_base_model_default="checkpoint",
+    )
+
+
+def test_unknown_base_model_checkpoint_setting_still_loses_to_file_type():
+    """The file-type signal stays first regardless of the setting."""
+    assert is_diffusion_model_download(
+        "checkpoint",
+        file_types=["Diffusion Model"],
+        base_model="SDXL 1.0",
+        unknown_base_model_default="checkpoint",
     )
 
 
@@ -35,9 +73,15 @@ def test_non_checkpoint_types_never_route_to_unet():
     )
 
 
-def test_empty_inputs_stay_on_checkpoint_roots():
-    assert not is_diffusion_model_download("checkpoint")
-    assert not is_diffusion_model_download("checkpoint", file_types=[], base_model="")
+def test_empty_base_model_follows_unknown_default():
+    assert is_diffusion_model_download("checkpoint")
+    assert is_diffusion_model_download("checkpoint", file_types=[], base_model="")
+    assert not is_diffusion_model_download(
+        "checkpoint",
+        file_types=[],
+        base_model="",
+        unknown_base_model_default="checkpoint",
+    )
 
 
 from py.services.download_routing import resolve_other_download_sub_type