[Improved, Fixed] Workable sunset cleanup

Improvements

• The setup checklist now appears for companies migrated from the old Workable plan and for legacy Flex users, guiding them to connect Indeed and choose a new pricing plan.
• The “Choose a plan” checklist step only marks complete once a paid plan is selected, giving a more accurate view of onboarding progress.
• Organizations that still have an active, billing Flex boost will continue to see the special flat-rate plan offer until their subscription period ends.

Fixes

• Restored the flat-rate plan migration offer for boosted jobs that were still charging but had disappeared after the Flex sunset.