refactor(assets): read content hash from the canonical hash field (#12638)

## Summary
The assets API exposes an asset's content hash as `hash`. An older
`asset_hash` field was a deprecated alias carrying the same value. This
PR moves the frontend fully onto `hash` and removes `asset_hash` from
the frontend entirely.

## Changes
- Read `asset.hash` (no `?? asset_hash` fallback) across the asset
consumers:
- `useMediaAssetActions` — widget-value variants + cloud-mode
stored-filename resolution
  - `assetsStore` — input-asset-by-filename map
  - `assetMetadataUtils.getAssetUrlFilename`
  - `missingMedia` resolver/scan and `missingModel` scan hash matching
  - `useComboWidget` / `useWidgetSelectItems`
- `assetPreviewUtil.findOutputAsset` now queries `/assets?hash=` instead
of the deprecated `?asset_hash=` param and matches on `a.hash`.
- Removed `asset_hash` from the zod asset schema and the local
`AssetRecord` type. Responses that still include the alias parse cleanly
— zod strips unknown keys — so the declared field protected nothing once
the reads were gone.
- Purged `asset_hash` from all test fixtures/mocks; tests key on the
canonical `hash`.

## Safety / rollout
The API currently emits **both** `hash` and `asset_hash` with identical
values, so reading `hash` is safe today. This is the frontend half of
retiring the alias; the backend stops emitting `asset_hash` only after
this ships and old bundles age out, so there is no window where the
field the UI reads is absent.

## Verification
- `pnpm typecheck`: clean.
- Affected unit tests pass (asset utils, store, media/model scans,
widget composables).
- `grep -rn asset_hash src/`: zero matches.

src/platform/missingModel/missingModelScan.ts

@@ -501,8 +501,7 @@ function isAssetInstalled(
 ): boolean {
   if (candidate.hash && candidate.hashType) {
     const candidateHash = `${candidate.hashType}:${candidate.hash}`
-    if (assets.some((a) => (a.hash ?? a.asset_hash) === candidateHash))
-      return true
+    if (assets.some((a) => a.hash === candidateHash)) return true
   }
 
   const normalizedName = normalizePath(candidate.name)