perf(config): limit symlink scan to first level for faster startup

Replace recursive directory traversal with first-level-only symlink scanning
to fix severe performance issues on large model collections (220K+ files).

- Rename _scan_directory_links to _scan_first_level_symlinks
- Only scan symlinks directly under each root directory
- Skip traversal of normal subdirectories entirely
- Update tests to reflect first-level behavior
- Add test_deep_symlink_not_scanned to document intentional limitation

Startup time reduced from 15+ minutes to seconds for affected users.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/config/test_symlink_cache.py

@@ -230,8 +230,58 @@ def test_new_symlink_triggers_rescan(monkeypatch: pytest.MonkeyPatch, tmp_path):
     assert normalized_external in second_cfg._path_mappings
 
 
-def test_removed_deep_symlink_triggers_rescan(monkeypatch: pytest.MonkeyPatch, tmp_path):
-    """Removing a deep symlink should trigger cache invalidation."""
+def test_removed_first_level_symlink_triggers_rescan(monkeypatch: pytest.MonkeyPatch, tmp_path):
+    """Removing a first-level symlink should trigger cache invalidation."""
+    loras_dir, settings_dir = _setup_paths(monkeypatch, tmp_path)
+
+    # Create first-level symlink (directly under loras root)
+    external_dir = tmp_path / "external"
+    external_dir.mkdir()
+    symlink = loras_dir / "external_models"
+    symlink.symlink_to(external_dir, target_is_directory=True)
+
+    # Initial scan finds the symlink
+    first_cfg = config_module.Config()
+    normalized_external = _normalize(str(external_dir))
+    assert normalized_external in first_cfg._path_mappings
+
+    # Remove the symlink
+    symlink.unlink()
+
+    # Second config should detect invalid cached mapping and rescan
+    second_cfg = config_module.Config()
+    assert normalized_external not in second_cfg._path_mappings
+
+
+def test_retargeted_first_level_symlink_triggers_rescan(monkeypatch: pytest.MonkeyPatch, tmp_path):
+    """Changing a first-level symlink's target should trigger cache invalidation."""
+    loras_dir, settings_dir = _setup_paths(monkeypatch, tmp_path)
+
+    # Create first-level symlink
+    target_v1 = tmp_path / "external_v1"
+    target_v1.mkdir()
+    target_v2 = tmp_path / "external_v2"
+    target_v2.mkdir()
+
+    symlink = loras_dir / "external_models"
+    symlink.symlink_to(target_v1, target_is_directory=True)
+
+    # Initial scan
+    first_cfg = config_module.Config()
+    assert _normalize(str(target_v1)) in first_cfg._path_mappings
+
+    # Retarget the symlink
+    symlink.unlink()
+    symlink.symlink_to(target_v2, target_is_directory=True)
+
+    # Second config should detect changed target and rescan
+    second_cfg = config_module.Config()
+    assert _normalize(str(target_v2)) in second_cfg._path_mappings
+    assert _normalize(str(target_v1)) not in second_cfg._path_mappings
+
+
+def test_deep_symlink_not_scanned(monkeypatch: pytest.MonkeyPatch, tmp_path):
+    """Deep symlinks (below first level) are not scanned to avoid performance issues."""
     loras_dir, settings_dir = _setup_paths(monkeypatch, tmp_path)
 
     # Create nested structure with deep symlink
@@ -242,46 +292,12 @@ def test_removed_deep_symlink_triggers_rescan(monkeypatch: pytest.MonkeyPatch, t
     deep_symlink = subdir / "styles"
     deep_symlink.symlink_to(external_dir, target_is_directory=True)
 
-    # Initial scan finds the deep symlink
-    first_cfg = config_module.Config()
+    # Config should not detect deep symlinks (only first-level)
+    cfg = config_module.Config()
     normalized_external = _normalize(str(external_dir))
-    assert normalized_external in first_cfg._path_mappings
-
-    # Remove the deep symlink
-    deep_symlink.unlink()
-
-    # Second config should detect invalid cached mapping and rescan
-    second_cfg = config_module.Config()
-    assert normalized_external not in second_cfg._path_mappings
+    assert normalized_external not in cfg._path_mappings
 
 
-def test_retargeted_deep_symlink_triggers_rescan(monkeypatch: pytest.MonkeyPatch, tmp_path):
-    """Changing a deep symlink's target should trigger cache invalidation."""
-    loras_dir, settings_dir = _setup_paths(monkeypatch, tmp_path)
-
-    # Create nested structure
-    subdir = loras_dir / "anime"
-    subdir.mkdir()
-    target_v1 = tmp_path / "external_v1"
-    target_v1.mkdir()
-    target_v2 = tmp_path / "external_v2"
-    target_v2.mkdir()
-
-    deep_symlink = subdir / "styles"
-    deep_symlink.symlink_to(target_v1, target_is_directory=True)
-
-    # Initial scan
-    first_cfg = config_module.Config()
-    assert _normalize(str(target_v1)) in first_cfg._path_mappings
-
-    # Retarget the symlink
-    deep_symlink.unlink()
-    deep_symlink.symlink_to(target_v2, target_is_directory=True)
-
-    # Second config should detect changed target and rescan
-    second_cfg = config_module.Config()
-    assert _normalize(str(target_v2)) in second_cfg._path_mappings
-    assert _normalize(str(target_v1)) not in second_cfg._path_mappings
 def test_legacy_symlink_cache_automatic_cleanup(monkeypatch: pytest.MonkeyPatch, tmp_path):
     """Test that legacy symlink cache is automatically cleaned up after migration."""
     settings_dir = tmp_path / "settings"