Add the async task that exports a dashboard's chart data to a multi-sheet xlsx.
Running as the requesting user, it walks charts in layout order, re-runs each
saved query context with the live filter state applied, streams results into a
constant-memory workbook, uploads to S3, and emails a pre-signed link. Charts
with no/invalid query context or a failing query are skipped and listed in the
email; all-skipped dashboards get a summary sheet. On failure (incl. soft
timeout) it emails the user and always cleans up the temp file. Registers the
task in CeleryConfig.imports. Adds unit tests for happy path, skips, summary,
upload failure, and timeout.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
