fix(controller): requeue at duration threshold expiry

[be-student]

Summary

• track the earliest durationThreshold expiry among pending failing pods
• requeue at that expiry when it is sooner than reconcileInterval
• apply a one-second floor to avoid hot loops when an expiry is imminent or passes during reconciliation
• document reconcileInterval as the maximum time between evaluations

Test plan

• go test ./internal/controller -run '^TestReconcile_RequeuesAtPendingDurationThreshold$|^TestThresholdRequeueAfter$' -count=1
• make ci

AI assistance

Codex was used to implement and validate this change. The implementation was checked with the repository's complete CI target.

Closes #69