Backport of 'Add Siphon ClickHouse cleanup Rake task'

What does this MR do and why?

This backports !248283 (merged) to GitLab 19.2.

It adds the gitlab:siphon:clean_clickhouse Rake task, which truncates the Siphon ClickHouse tables so an initial snapshot can be replayed from scratch (guarded by FORCE_CLEAN_SIPHON_TABLES_IN_CLICKHOUSE=true). The self-managed Orbit deployment documentation relies on this task for the data-pipeline re-snapshot step, so it must reach a released 19.2 patch for design-partner onboarding.

The cherry-pick was clean (no conflicts).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
  • This is not a bug fix. It is backported under the approved exception for Orbit backports (including Siphon), because gitlab:siphon:clean_clickhouse is required by the self-managed Orbit deployment documentation and must be available in a released 19.2 patch for design-partner onboarding.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
  • Merged to master in !248283 (merged) on 2026-08-03 and deployed to GitLab.com.
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.