fix(scanner): compare model roots by identity, not by spelling (#1146)

ComfyUI hands its model folders over in the host's own spelling --
`utils/extra_config.py` runs every configured path through `os.path.normpath()`,
which is all backslashes on Windows, and `main.py` registers `<output>/loras`
unconditionally for its save nodes. `Config` kept that spelling verbatim while
`_prepare_*_paths()` normalized the live root lists to forward slashes, and the
scanner compared the two as raw strings. On Windows every configured root
therefore looked like a root that was not in the live list: the Refresh menu
listed each directory twice (the twin showing "0 models"), the completion toast
claimed reachable roots were "currently inaccessible" and kept their whole cache
count as `kept_unreachable`, and the settings sync persisted host-registered
directories that were never created.

The duplication was not only cosmetic. `_UnreachablePaths.add()` normalizes the
prefix it is given, so the configured twin of a live root registered exactly the
live root's prefix as unreadable: in the prune phase every cached-but-missing
entry under it matched and was "kept", so models deleted from disk were never
removed from the cache again.

* `py/utils/root_paths.py` holds the single `normalize_root_identity()`
  (separator style, redundant/trailing separators, and case where the filesystem
  folds it: whole paths on Windows, drive letters everywhere). `config.py` and
  `settings_manager.py` each carried a private copy and `model_scanner.py`
  compared raw strings.
* `ModelScanner` compares root identities in `get_configured_model_roots()`,
  `describe_model_roots()` (dedupe + `available`), the reconcile's
  `configured_only` list and the `ReconcileScope.roots` membership test, so one
  directory is one root whatever the host called it. `_normalized_root_prefix()`
  is built on the same identity, and an empty prefix no longer matches every
  path.
* A configured root that never made it into the live list is reported only while
  it still holds cached entries -- in the menu and in `skipped_roots` /
  `unavailable_paths` alike. A directory that neither exists nor was ever scanned
  has nothing to keep, so `<output>/loras` stops being an unavailable root.
* The startup sync drops saved roots that neither exist nor hold cached entries
  (`PersistentModelCache.roots_with_cached_entries()`, one scan per model type,
  separator-agnostic and wildcard-escaped). A root named by the library's
  `default_*_root` is kept even when empty: dropping it would let the next boot
  "repair" the default onto another root. When the cache cannot be consulted,
  every root is kept.
* `GET /scan?roots=` validates requested roots by identity too, so the browser
  extension or a script passing the spelling settings.json holds selects the root
  instead of answering 400 "Unknown model root(s)".

Verified: 8 of the new tests fail against the pre-fix source (checked with the
fix stashed); 3798 passed, 7 skipped, 11 deselected. The probe costs 71ms for a
100k-model library with seven missing roots (357ms when probing root by root).

py/routes/handlers/model_handlers.py

@@ -55,6 +55,7 @@ from ...utils.civitai_utils import resolve_license_payload
 from ...utils.file_utils import calculate_sha256
 from ...utils.metadata_manager import MetadataManager
 from ...utils.paid_access import is_early_access_deadline_active
+from ...utils.root_paths import normalize_root_identity
 from ...utils.sidecar_paths import get_metadata_path
 from ...utils.url_utils import relative_root_prefix
 
@@ -1170,7 +1171,18 @@ class ModelQueryHandler:
 
             if requested_roots:
                 configured = self._service.get_model_roots()
-                unknown = [root for root in requested_roots if root not in configured]
+                # Compare by identity, not by string: callers may pass the host's
+                # own spelling of a root (backslashes or a differently cased drive
+                # letter on Windows, as settings.json and ComfyUI show it), which
+                # names the same directory the live list holds forward-slashed.
+                known_roots = {
+                    normalize_root_identity(root) for root in configured
+                }
+                unknown = [
+                    root
+                    for root in requested_roots
+                    if normalize_root_identity(root) not in known_roots
+                ]
                 if unknown:
                     return web.json_response(
                         {"error": "Unknown model root(s)", "roots": unknown}, status=400