Add CloudWatch alarm for ApprovalMetricsPublisher EMF rate ceiling

Start by reading cdk/src/handlers/approval-metrics-publisher.ts and verify the exact skipped-count metric name, then inspect cdk/src/constructs/approval-metrics-publisher-consumer.ts for existing CloudWatch alarms and construct tests. Add the alarm with a documented threshold and verify its presence in the construct test; link a runbook only if one exists.

Follow-up from PR #88 — observability blind spot in the metrics publisher path.

Functional description

The ApprovalMetricsPublisher Lambda emits CloudWatch metrics via the EMF (Embedded Metric Format) pattern: instead of calling PutMetricData directly, it logs structured JSON that CloudWatch Logs auto-extracts into metrics. EMF has its own per-account throttle ceiling — 100 EMF metric writes per second per account. Cross that ceiling and CloudWatch silently drops metrics with no error visible to the Lambda.

The publisher already self-rate-limits and emits an internal MetricEmitSkipped count when it self-limits, but there's no alarm on MetricEmitSkipped > 0. So if approvals scale to where the publisher hits the ceiling, the dashboard quietly underreports without anyone noticing.

This pairs with issue #4 (DLQ alarms): both are about "the metrics path is broken but the dashboard still shows old data." Filing as separate issues since the threshold-tuning conversation will be different (DLQ count = 1 is the right alarm; EMF skipped count needs more thought — burst spikes can be normal).

User-visible impact:

• Approval-volume metrics under-report at high load. Operators see lower ApprovalRequestCount than reality.
• No signal to "scale up the Lambda's batch size" or "adjust EMF emission cadence."
• Capacity-planning conversations rely on CloudWatch numbers that are silently lower than actual.

Technical context

Where the metric is already emitted:

• cdk/src/handlers/approval-metrics-publisher.ts — search for MetricEmitSkipped (or similar; the exact name needs verification). The publisher's self-rate-limit logic increments this when it skips an EMF write to stay under the ceiling.

What's missing:

• cloudwatch.Alarm on MetricEmitSkipped > <threshold> over a sensible window.
• Threshold tuning: a low threshold (1 over 5 min) will fire on legitimate burst spikes; a high threshold (>100 over 1 hour) misses sustained problems. Recommend starting at "10 over a 15-minute window" as a conservative baseline; tune after observing real traffic.

Why EMF instead of PutMetricData:

• EMF batches 100x cheaper than PutMetricData for the same observability.
• The trade-off is the global per-account ceiling. PutMetricData has its own throttle but it's per-region per-account at a much higher number.
• The publisher's choice of EMF is the right one; this issue isn't asking to change that.

Proposed fix

Add the alarm in cdk/src/constructs/approval-metrics-publisher-consumer.ts:

new cloudwatch.Alarm(this, 'MetricEmitSkippedAlarm', {
  metric: new cloudwatch.Metric({
    namespace: 'ABCA/Cedar-HITL',
    metricName: 'MetricEmitSkipped',
    period: Duration.minutes(15),
    statistic: 'Sum',
  }),
  threshold: 10,
  evaluationPeriods: 1,
  comparisonOperator: ComparisonOperator.GREATER_THAN_OR_EQUAL_TO_THRESHOLD,
  alarmDescription: 'ApprovalMetricsPublisher hit the EMF rate ceiling and dropped metrics; consider increasing batch size or reducing per-event metric volume',
  treatMissingData: TreatMissingData.NOT_BREACHING,
});

Acceptance criteria

• Alarm exists on MetricEmitSkipped (or whatever the existing skipped-count metric is named — verify the name in approval-metrics-publisher.ts first)
• Threshold is documented in the alarm description with rationale
• Construct test verifies alarm presence
• If a runbook exists for "publisher hit EMF ceiling," alarmDescription links to it

Out of scope

• Refactoring the publisher to use PutMetricData (different cost/perf profile, separate decision).
• Adaptive batch-size tuning (publisher could observe its own skip rate and adjust; that's a separate enhancement).
• Multi-region failover for metric publishing (out of scope for ABCA today).

References

• cdk/src/handlers/approval-metrics-publisher.ts
• cdk/src/constructs/approval-metrics-publisher-consumer.ts
• AWS EMF reference: https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Embedded_Metric_Format_Specification.html
• Per-account EMF ceiling: https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_limits.html
• Companion issue: #4 (DLQ alarms, same observability gap shape)