feat(scanner): report and re-admit model roots that are unavailable (#1108)

`Config._dedupe_existing_paths()` drops roots whose directory does not exist
when the root list is built, so a drive that is switched off while LM starts was
not "an offline root" — it was not a root at all. The Refresh ▾ menu had no row
for it in either state, a full refresh reported nothing (its cached entries
survived only because they fall outside every configured prefix), and plugging
the drive back in changed nothing until the next restart.

* `Config` now records the configured (existence-unfiltered) paths per model
  type — from ComfyUI's list in plugin mode, from the active library snapshot in
  standalone mode, where the host mock filters non-existent paths itself — and
  exposes them through `configured_roots_for()`.
* `ModelScanner.describe_model_roots()` appends configured-but-unavailable roots
  with a live `reachable`, an `available` flag and their cached entry count, so
  `/roots` (and therefore the menu) offers a row in both states: greyed while the
  directory is missing, normal and clickable once it is back.
* `_reconcile_cache()` reports them as `skipped_roots` / `unavailable_paths` with
  reason `root_unavailable` and counts their cached entries in `kept_unreachable`,
  which is what makes the "N models kept" toast appear in the startup-offline
  case. Report-only: they stay outside the scan scope, so nothing about pruning
  changes. A root-scoped scan stays quiet about roots it was not asked about, and
  a folder-scoped scan only mentions a missing root that holds cached entries of
  that folder.
* `Config.admit_configured_roots()` re-runs the per-type prepare helpers against
  the configured paths and APPENDS what is readable now (plus the
  checkpoint/unet/other side maps), then refreshes the preview allowlist, so a
  drive plugged in mid-session can be scanned without restarting. Append-only and
  order-preserving by design: removing a root mid-session would let a later
  settings save persist the loss, and re-sorting would move `*_roots[0]`, which
  derives the recipes directory and the usage-stats file location. `_roots[0]`
  never moves.
* `/roots` and `/scan` admit first, so `describe_model_roots()` stays a pure read
  and a caller that never opens the menu (the browser extension) still gets a
  drive that came back.

Verified on the sandbox with drive-Z switched off before startup: `/roots`
reports `drive-Z reachable=false available=false models=60`, a full refresh
returns `skipped_roots=[drive-Z root_unavailable]`,
`unavailable_paths=[{... kept: 60}]`, `kept_unreachable=60`, and all 432 models
stay cached. Renaming the drive back while the server runs admits it
(`available=true`) and `GET /scan?roots=<drive-Z>` walks it with no restart.
3714 passed, 7 skipped; frontend 1495 passed (149 files); vue widgets 96 passed.

py/services/base_model_service.py

@@ -954,9 +954,17 @@ class BaseModelService(ABC):
         return self.scanner.last_reconcile_summary
 
     def describe_model_roots(self) -> List[Dict[str, Any]]:
-        """Describe configured roots (label / reachability / cached count)."""
+        """Describe configured roots (label / reachability / cached count).
+
+        Pure read: callers that want a drive which was plugged in after startup to
+        appear as a normal row call :meth:`refresh_model_roots` first.
+        """
         return self.scanner.describe_model_roots()
 
+    def refresh_model_roots(self) -> List[str]:
+        """Admit configured roots that became readable again (append-only)."""
+        return self.scanner.refresh_model_roots()
+
     async def get_model_info_by_name(self, name: str):
         """Get model information by name"""
         return await self.scanner.get_model_info_by_name(name)