Define completion criteria for onboarding checklist content steps

[rchlfryn]

Summary

The onboarding checklist currently checks completion by querying whether expected content exists (e.g., pages with specific slugs). This means if a tenant admin deletes or renames a provisioned page, the checklist reverts to "Incomplete" even though provisioning originally succeeded.

Example: If a tenant renames their donate page slug (as Sierra did), the checklist shows the page as missing.

See discussion: #974 (comment)

Options to consider

1. Track provisioning history — Store the IDs of pages created during provisioning, so completion reflects "was successfully created" rather than "currently exists"
2. Store a completion flag — Add a field on the Tenants collection (e.g., provisioningComplete: true) that gets set once provisioning succeeds, independent of current content state
3. Keep current behavior — Checklist reflects the live state, which is useful for detecting accidental deletions but noisy for intentional changes

Acceptance Criteria

• Decide on the desired definition of "completed" for each content step
• Implement the chosen approach
• Update tests to reflect the new behavior