PFX-WB-038: add hidden caption semantics for raw import metadata table

[peshay]

Summary

• add an app-shell visually hidden <caption> to #raw-import-item-review-metadata-table
• keep the change limited to read-only UI accessibility semantics on the raw import item review page
• add a LiveView test assertion for the hidden caption text and class

Scope check

• no changes to import processing, document handling, persistence, routing, or write paths

Verification

• mix format
• mix test (fails in this runner: PostgreSQL unavailable at 127.0.0.1:5432)