CHW-4502 UsagePlanDefaultConflict
another pricing plan is already marked as the default; clear the existing default before promoting a new one
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Default message: another pricing plan is already marked as the default; clear the existing default before promoting a new one
HTTP status:409(client)
Problem. POST /api/usage/plans or PUT /api/usage/plans/{id}
returned 409. Cause. The partial unique index
uq_pricing_plans_default rejects more than one row with
is_default=1. Fix. Demote the existing default plan first
(PUT /api/usage/plans/{currentDefault} with isDefault=false), then
retry. HTTP 409.