Load images and image aliases through bulk get

image_loader (image.search_images) and image_alias_loader (image.search_aliases) read through SUPERADMIN-gated global searches. Switch images to an entity partial bulk get and aliases to a field row bulk get.

## Success Criteria
- [ ] images load through an entity partial bulk get
- [ ] image aliases load through batch_load_fields
- [ ] an unreadable id errors only its own field; a missing id resolves to None
- [ ] pants test passes for affected packages

JIRA Issue: BA-7891

Start by locating image_loader.image.search_images and image_alias_loader.image.search_aliases, then trace the existing SUPERADMIN-gated global searches and the bulk-loading helpers they can use. Update the loaders so images use an entity partial bulk get and aliases use batch_load_fields, preserving per-field unreadable-id errors and None for missing ids; run the pants tests for the affected packages.

Written by the indexing model from the issue text.