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.

tests/services/test_download_manager_other.py

@@ -254,6 +254,7 @@ async def test_download_rejects_other_when_feature_disabled(
 
     assert result["success"] is False
     assert "disabled" in result["error"].lower()
+    assert result["reason"] == "other_models_disabled"
 
 
 @pytest.mark.asyncio
@@ -271,6 +272,7 @@ async def test_default_paths_reject_switched_off_sub_type(
 
     assert result["success"] is False
     assert "disabled" in result["error"].lower()
+    assert result["reason"] == "other_sub_type_disabled"
 
 
 @pytest.mark.asyncio
@@ -516,6 +518,7 @@ async def test_default_paths_errors_when_sub_type_root_unconfigured(
 
     assert result["success"] is False
     assert "controlnet" in result["error"]
+    assert result["reason"] == "other_no_default_root"
     assert execute_mock.await_count == 0
 
 
@@ -537,6 +540,7 @@ async def test_default_paths_errors_when_sub_type_undecidable(
 
     assert result["success"] is False
     assert "sub-type" in result["error"]
+    assert result["reason"] == "other_sub_type_undecidable"
     assert execute_mock.await_count == 0
 
 
@@ -559,6 +563,72 @@ async def test_civarchive_source_same_payload_shape(
     assert captured["model_type"] == "other"
 
 
+@pytest.mark.asyncio
+async def test_other_failure_reasons_are_machine_readable(
+    monkeypatch, scanners, metadata_provider, tmp_path
+):
+    """Contract C4: every other-type default-path failure carries a ``reason``.
+
+    The companion browser extension binds to ``reason`` and only falls back to
+    substring matching for backends that predate the field, so the exact values
+    below must not drift.
+    """
+    expected_reasons = {
+        "disabled": "other_models_disabled",
+        "sub_type_disabled": "other_sub_type_disabled",
+        "no_default_root": "other_no_default_root",
+        "undecidable": "other_sub_type_undecidable",
+    }
+    reasons: dict[str, str] = {}
+
+    manager = DownloadManager()
+
+    # 1. Master switch off.
+    metadata_provider.payload = _other_payload("VAE")
+    get_settings_manager().settings["enable_other_models"] = False
+    disabled = await manager.download_from_civitai(
+        model_version_id=99, save_dir=str(tmp_path)
+    )
+    reasons["disabled"] = disabled["reason"]
+    assert disabled["error"].strip()
+
+    get_settings_manager().settings["enable_other_models"] = True
+
+    # 2. Resolved sub_type not enabled.
+    get_settings_manager().settings["enabled_other_sub_types"] = ["upscaler"]
+    sub_type_disabled = await manager.download_from_civitai(
+        model_version_id=99, use_default_paths=True
+    )
+    reasons["sub_type_disabled"] = sub_type_disabled["reason"]
+    assert sub_type_disabled["error"].strip()
+
+    get_settings_manager().settings["enabled_other_sub_types"] = [
+        "vae",
+        "upscaler",
+        "text_encoder",
+        "clip_vision",
+        "controlnet",
+    ]
+
+    # 3. Sub_type resolved but no default root configured.
+    metadata_provider.payload = _other_payload("Controlnet")
+    no_default_root = await manager.download_from_civitai(
+        model_version_id=99, use_default_paths=True
+    )
+    reasons["no_default_root"] = no_default_root["reason"]
+    assert no_default_root["error"].strip()
+
+    # 4. Neither model.type nor file types map to a sub_type.
+    metadata_provider.payload = _other_payload("Other")
+    undecidable = await manager.download_from_civitai(
+        model_version_id=99, use_default_paths=True
+    )
+    reasons["undecidable"] = undecidable["reason"]
+    assert undecidable["error"].strip()
+
+    assert reasons == expected_reasons
+
+
 def test_build_metadata_for_resume_uses_other_metadata():
     manager = DownloadManager()
     metadata = manager._build_metadata_for_resume(