[Feature] Convert CSV imports into async jobs with dry-run preview, progress, cancel, and rollback support

[richardcmckinney]

Problem

The import route says “Upload and queue CSV import job” but processes the import inline. Large imports should not depend on a single request lifecycle, and admins need a preview before mutating production data.

Proposed solution

Implement CSV imports as background jobs with an import record. Add dry-run validation, row-level preview, progress tracking, cancellation, resumable processing, and a rollback/export plan for created records.

Acceptance criteria

• Upload creates an import job and returns a job ID.
• Admins can run a dry-run validation before applying changes.
• UI shows progress, imported count, skipped count, error count, and created tags.
• Admins can cancel a pending or running import.
• Failed rows can be downloaded as a CSV with error messages.
• Completed imports produce an audit event and a summary report.
• Import processing does not require the browser request to stay open.