feat(backend): make model existence checks other-aware

Implements the backend slice (B1-B7) of
lm-civitai-extension/docs/other-models-support.md, which lets the companion
browser extension detect, badge and download the opt-in Other Models types
(VAE / upscaler / text encoder / CLIP vision / ControlNet).

ModelLibraryHandler:
- _normalize_model_type() learns the CivitAI other aliases (vae, upscaler,
  textencoder, clip, clipvision, controlnet, other) and maps them to "other".
- _get_scanner_for_type() resolves "other" through the other scanner, but only
  while enable_other_models is on, so model-versions-status and
  model-version-download-status keep their legacy 400 when the feature is off.
- check_model_exists() / check_models_exist() consult the other scanner last
  (lora -> checkpoint -> embedding -> other) and report modelType "other".
  With the feature disabled both endpoints stay byte-identical to before and
  the other scanner is never touched.

DownloadManager:
- The four other-type default-path failures now carry a machine-readable
  "reason" (contract C4): other_models_disabled, other_sub_type_disabled,
  other_no_default_root, other_sub_type_undecidable. The user-facing "error"
  strings are unchanged; the key is additive and reaches the client because
  both download endpoints pass the result dict through verbatim.

Tests cover the opt-in on/off branches for both existence endpoints, mixed
lora + other ids in the batch endpoint, the CivitAI alias acceptance and the
400 regression for unknown types, and the exact reason/error pairs for all
four download failure modes.

py/services/download_manager.py

@@ -1534,6 +1534,9 @@ class DownloadManager:
                             "Settings > Library before downloading VAE, upscaler, "
                             "text encoder or CLIP files."
                         ),
+                        # Machine-readable failure code consumed by the companion
+                        # browser extension (docs/other-models-support.md C4).
+                        "reason": "other_models_disabled",
                     }
                 model_type = "other"
             else:
@@ -1793,6 +1796,7 @@ class DownloadManager:
                                     f"disabled in settings. Please pick a destination "
                                     f"folder explicitly instead of using default paths."
                                 ),
+                                "reason": "other_sub_type_disabled",
                             }
                         default_path = (
                             default_other_roots.get(other_sub_type)
@@ -1805,17 +1809,20 @@ class DownloadManager:
                                     f"No default root configured for other-model "
                                     f"sub-type '{other_sub_type}'"
                                 )
+                                reason = "other_no_default_root"
                             else:
                                 detail = (
                                     "Could not determine the other-model sub-type "
                                     "from the model metadata"
                                 )
+                                reason = "other_sub_type_undecidable"
                             return {
                                 "success": False,
                                 "error": (
                                     f"{detail}. Please pick a destination folder "
                                     f"explicitly instead of using default paths."
                                 ),
+                                "reason": reason,
                             }
                         save_dir = default_path