ALL-1872 PR-1: Backend - bulk upload job results CSV export endpoint

Add GET /v1/bulk-upload/jobs/:jobId/download endpoint that streams a results CSV. Columns: all original CSV columns from BulkImportJobRow.rowData + Status (SUCCESS/ERROR/SKIPPED) + Error Message + Entity ID columns (customerId, siteId, deviceId from BulkImportEntity join). Use streaming CSV writer (csv-stringify or similar already in repo). Auth: workspace scoping. Branch: wilbo/all-1872-bulk-upload-download. Worktree: /home/agent/agents/wilbo/mono-all-1872-bulk-upload-download. PR-2 (frontend Download Results button in JobStatusCard + JobHistoryTable action column) is a follow-up fleet-task after backend merges.