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.

py/services/download_manager.py

@@ -1523,7 +1523,11 @@ class DownloadManager:
                 return {"success": False, "error": "Failed to fetch model metadata"}
 
             model_type_from_info = version_info.get("model", {}).get("type", "").lower()
-            if model_type_from_info == "checkpoint":
+            # CivitAI ModelType has no model-level diffusion variant: DiT
+            # models are uploaded as "Checkpoint" or "UNet". Both go through
+            # the checkpoint branch so the standard diffusion routing chain
+            # (file type -> baseModel lists -> unknown default) applies.
+            if model_type_from_info in ("checkpoint", "unet"):
                 model_type = "checkpoint"
             elif model_type_from_info in VALID_LORA_TYPES:
                 model_type = "lora"