Flex: per-capability spend caps for Credits on the reservation page

This issue is the public record for the GitLab 19.3 release note. Implementation was tracked in internal issues.

GitLab Flex now supports per-capability spend controls for Credits-based add-ons on the reservation page in the Customers Portal.

When adjusting a monthly Flex reservation, a billing account manager can set a spend cap for each credit-based capability: Restricted (no overage past the reservation), Usage cap (a bounded amount of overage), or Unlimited. When a capped capability reaches its cap, its usage stops for the rest of the billing period. Capabilities without a cap keep running.

Shipped in GitLab 19.3. Documentation: https://docs.gitlab.com/subscriptions/gitlab_flex/#spend-caps