Fix Actions minutes timeout cap

[espadonne]

Summary

• cap billable Actions minutes at each job's timeout_minutes
• apply the same timeout cap to Actions usage/performance UI metrics
• add regression coverage for stale cancelled jobs whose wall-clock gap exceeds their timeout

Tests

• GOCACHE=/private/tmp/shithub-go-cache SHITHUB_TEST_DATABASE_URL='postgres://shithub:shithub_dev@127.0.0.1:5432/postgres?sslmode=disable' go test ./internal/billing ./internal/actions/...
• make build