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.

py/utils/constants.py

@@ -371,6 +371,7 @@ DIFFUSION_MODEL_BASE_MODELS = frozenset(
         # Other diffusion models
         "Ernie",
         "Ernie Turbo",
+        "MiniMax H3",
         "Nucleus",
         "Qwen",
         "ZImageBase",
@@ -380,6 +381,39 @@ DIFFUSION_MODEL_BASE_MODELS = frozenset(
     ]
 )
 
+# baseModel values from CivitAI that are true full checkpoints (loaded via
+# CheckpointLoaderSimple in ComfyUI). New DiT families appear on CivitAI all
+# the time, so download routing inverts the fallback: anything NOT in this
+# closed set (and not a known diffusion model) is treated as a diffusion
+# model by default (see py/services/download_routing.py).
+# "Pony V7" is deliberately excluded: it is not an SDXL-derivative full
+# checkpoint, so it follows the unknown-base-model default (diffusion).
+CHECKPOINT_BASE_MODELS = frozenset(
+    [
+        # Stable Diffusion 1.x
+        "SD 1.4",
+        "SD 1.5",
+        "SD 1.5 LCM",
+        "SD 1.5 Hyper",
+        # Stable Diffusion 2.x
+        "SD 2.0",
+        "SD 2.1",
+        # Stable Diffusion 3.x
+        "SD 3",
+        "SD 3.5",
+        "SD 3.5 Medium",
+        "SD 3.5 Large",
+        "SD 3.5 Large Turbo",
+        # SDXL and its full-checkpoint derivatives
+        "SDXL 1.0",
+        "SDXL Lightning",
+        "SDXL Hyper",
+        "Pony",
+        "Illustrious",
+        "NoobAI",
+    ]
+)
+
 # Supported baseModel values for download exclusion settings.
 # Keep this aligned with static/js/utils/constants.js, excluding the generic "Other" value.
 SUPPORTED_DOWNLOAD_SKIP_BASE_MODELS = frozenset(