Add warm capacity observability

[bill-ph]

Summary

• add Kubernetes warm-capacity metrics for foreground misses, recent demand, base/dynamic/effective targets, ready/spawning warm workers, global headroom, and reconciliation spawn outcomes
• wire dynamic warm-capacity target and worker-state metrics into the janitor path without adding foreground configstore reads
• add reason-aware capacity reject metric labels plus OpenTelemetry/log attributes for miss reason and retry hint
• add configstore aggregation for cluster-wide ready/spawning neutral warm workers by image scope
• update the dynamic warm-capacity plan with the PR E observability scope

Tests

• go test ./controlplane
• go test ./controlplane/configstore
• go test -tags kubernetes ./controlplane
• go test -tags kubernetes ./controlplane ./cmd/duckgres-controlplane
• go test -run 'TestListWarmCapacityWorkerStatsPostgres|TestRecordWarmCapacityMissAggregatesByBucketScopeAndReasonPostgres|TestListWarmCapacityMissesSinceAggregatesByScopeAndReasonPostgres' ./tests/configstore
• golangci-lint run