[DwC export]: Report cache build progress to UI and update "last exported" timestamp

[grantfitzsimmons]

Goal

During cache table building, report progress to the UI in real time. On completion, update the mapping's "last exported" timestamp.

Background

Cache builds can be long-running for large collections. Users need feedback that the process is running and when it completes.

Acceptance Criteria

• A progress indicator (e.g., loading bar or percentage) is shown in the UI during cache building.
• Progress updates are pushed from the backend (via polling or the existing worker notification mechanism).
• On successful completion, the schemamapping record's "last exported" timestamp is updated.
• On failure, an error message is displayed and the timestamp is not updated.