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.

locales/ru.json

@@ -353,6 +353,14 @@
             "placeholder": "Оставьте пустым, чтобы использовать aria2c из PATH"
         },
         "aria2HelpLink": "Узнайте, как настроить сервер загрузки aria2",
+        "unknownBaseModelRouting": {
+            "label": "[TODO: Translate] Unknown base model routing",
+            "help": "[TODO: Translate] Decides where checkpoint downloads go when CivitAI reports a base model that is neither a known checkpoint (SD 1.x/2.x/3.x, SDXL, Pony, Illustrious, NoobAI) nor a known diffusion model. New diffusion architectures appear frequently, so routing them to diffusion models is usually correct.",
+            "options": {
+                "diffusionModel": "[TODO: Translate] Diffusion models (recommended)",
+                "checkpoint": "[TODO: Translate] Checkpoints"
+            }
+        },
         "civitaiHostBanner": {
             "title": "Доступна настройка хоста CivitAI",
             "content": "Теперь CivitAI использует civitai.com для контента SFW и civitai.red для контента без ограничений. В настройках можно изменить, какой сайт открывать по умолчанию.",