Add Cells::Claimable to PagesDomain for domain claiming in Topology Service

What does this MR do and why?

This MR implements Step 1 of the GitLab Pages Cells work (#473017): claiming Pages custom domains and unique domains in the Topology Service so that the domain→cell mapping is available for routing.

Rollout plan

3. Enable cells_claims_pages_domains and cells_claims_project_settings_pages_unique_domains flags (Phase 1 — live claiming)
4. Enable cells_claims_verification_worker_pages_domain and cells_claims_verification_worker_project_setting flags (Phase 2 — backfill and drift correction)

References

• Issue: #473017
• Cells claims documentation: https://docs.gitlab.com/development/cells/claims/
• Topology Service bucket/source types: gitlab-org/cells/topology-service!538 (merged)
• Extracted migration: !243452 (merged)

Screenshots or screen recordings

N/A — backend-only change.

How to set up and validate locally

1. Enable the Cells GDK setup and the cells_unique_claims feature flag
2. Enable the relevant feature flag(s):

   Feature.enable(:cells_claims_pages_domains)
   Feature.enable(:cells_claims_project_settings_pages_unique_domains)

3. Create a Pages custom domain (or a unique domain via project settings) in the UI or via API
4. Verify the claim was created:

   gdk psql -d topology_service -c "SELECT * FROM claims WHERE bucket_type IN (17, 18);"

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.