deploy: gate connection budgets during progressive delivery

[TheSentinel454]

Problem

Buzz will adopt Kargo promotion and Argo Rollouts verification, but that infrastructure is not in place yet. Deployment-wide PostgreSQL connection-budget enforcement belongs there rather than in relay startup: a pod does not have authoritative rollout/HPA context, and rejecting startup on a theoretical ceiling can deadlock recovery.

Follow-up

After progressive delivery is established:

• Expose a deterministic per-database-target connection-budget calculation from rendered Buzz configuration.
• In Kargo destination-environment verification, calculate steady and rollout-surge ceilings from replica limits, surge, and explicit per-role pool maxima.
• Reject intrinsically invalid configuration; initially report theoretical overcommit as promotion evidence/warning, with an opt-in strict promotion gate after production baselines exist.
• Add Argo Rollouts analysis over observed pool pressure: open/in-use connections, acquisition waits/timeouts, budget utilization, and database saturation/errors.
• Keep relay startup non-blocking for deployment-wide theoretical overcommit. Runtime owns role-tagged metrics and local consistency only.
• Add post-promotion monitors for drift from HPA, database capacity, and non-Buzz consumers.

Dependencies

• Progressive-delivery/Kargo/Argo Rollouts infrastructure is available.
• Pool roles and explicit per-role maxima from buzz-db: centralize pool configuration #11 and perf: connection pool budget accounting across relay process #27 are implemented and observable.

Acceptance criteria

• Promotion uses the destination environment's rendered values, not the source environment's assumptions.
• Evidence records per-role, per-target, replica, and surge inputs plus calculated steady/surge ceilings.
• Unsafe observed pressure can pause/abort a canary without requiring new pods to fail startup.
• Tests pin target grouping, optional roles, fixed replicas, HPA maximum, and rollout surge.

Generated by Elrond (AI agent).