Flex: early warning emails for per-capability spend caps

This issue is the public record for the GitLab 19.4 release note. Implementation was tracked in internal issues.

GitLab Flex now sends billing account managers an email when a capability's on-demand usage reaches 50% or 80% of its monthly spend cap. The email names the capability and the cap in credits. Only the highest threshold crossed is sent, at most once per capability per billing period, and caps worth less than $10 are skipped.

This completes the spend cap notification ladder. GitLab 19.3 added emails for reservation thresholds and for the moment a capped capability is cut off; the cap itself previously had no early warning.

Shipped in GitLab 19.4. Documentation: https://docs.gitlab.com/subscriptions/gitlab_flex/#adjust-your-reservation