Feature/8 move multi selection processing to backend

[connykaelinvisol]

What I did
Added backend support for batch asset mutations (delete, tag, untag, assign collection, update) so that multi-selection operations are processed in a single GraphQL request instead of sending individual mutations per asset from the frontend. Additionally, standardized all single-asset mutations to return MutationResult instead of a mix of Asset and MutationResult, and extracted shared validation logic into reusable helper methods.

How I did it (main changes and additional fixes)
Backend:

• Added resolveAsset() and resolveTag() helper methods to AssetMutator to eliminate duplicated asset/tag resolution and validation logic across 9+ methods
• Standardized updateAsset, tagAsset, untagAsset, and setAssetTags to return MutationResult instead of ?Asset — aligning them with deleteAsset, setAssetCollections, and editAsset which already used this pattern
• Added 5 batch mutation methods to AssetMutator: deleteAssets, tagAssets, untagAssets, assignAssetsToCollection, updateAssets — each iterates server-side with per-asset error handling
• Renamed AssetIdentities::$collections to $identities to fix a misleading property name
• Added a TYPE_ALIASES mapping in Resolver.php to handle the irregular plural AssetIdentity → AssetIdentities (the resolver's default pluralization appends 's', producing AssetIdentitys which doesn't match)

Frontend:

• Created 5 new GraphQL mutation definitions (deleteAssets, tagAssets, untagAssets, assignAssetsToCollection, updateAssets) and 5 new hooks (useDeleteAssets, useTagAssets, useUntagAssets, useAssignAssetsToCollection, useUpdateAssets)
• Updated 4 existing single-asset mutation definitions (updateAsset, tagAsset, untagAsset, setAssetTags) to request { success messages } instead of ...AssetProps and 4 existing hooks (useUpdateAsset, useSetAssetTags, useTagAsset, useUntagAssetById) to handle MutationResult responses with refetchQueries instead of optimistic responses
• Added getFailedAssetLabelsFromResults to useFailedAssetLabels for mapping batch results to failed asset labels
• Updated DeleteAssetButton, TagSelectBoxMulti, CollectionSelectBox, and PropertyInspector to use the batch hooks instead of Promise.allSettled with individual mutations

[connykaelinvisol]

Within my work on this feature I noticed that there is a response handling mix for the single asset actions. I therefore standardized it to use MutationResult in every action. Even though this requires an additional refetchQueries I think the performance loss is neglectable (and quite low). The upside is a more consistent result handling.