feat: expand the Finding model for a real triage queue (id, pivot, cost, severity, dedup/snooze)

[Bccorb]

Problem

The Finding model is too thin for a triage accelerator: no stable resource_id/ARN, no executable CLI/console pivot (just free-text next_step), no cost estimate on waste findings, only High/Medium severity, and no dedup/acknowledge/snooze/persistence, so the queue never converges.

Evidence

• src/models/finding.rs:56 (fields), :1 (severity), :40 (FindingRoute duplicates ActiveView, omits ECS)
• fresh rebuild each refresh: src/app/mod.rs:1350

Impact

The single biggest gap versus the stated triage/waste-catalog vision. Also related to the stale-findings bug #13.

Proposed fix

Add resource id/region, an executable pivot (reuse DescribableResource cli/console), an optional monthly-cost estimate for waste, a fuller severity scale, and persistence for acknowledge/snooze keyed by resource id.

Acceptance

• Findings carry a stable id and an executable pivot
• Waste findings carry a cost estimate
• Operator can acknowledge/snooze a finding across refreshes

[Bccorb]

Re-checked against current main. Two of four acceptance criteria are met.

Findings carry a stable id — done. Finding gained rule and resource_id, and Finding::key() combines them with the region. It is used for real deduplication in build_findings, with tests for cross-region distinctness. Four rules deliberately carry no id: the alarm-coverage rule reasons about service areas, and three overview fallbacks fire from summary counters when a detail list could not be fetched, so there is no resource to point at.

Fuller severity scale — done. LOW was added and the category sort now ranks by urgency rather than comparing labels alphabetically, which had been placing Hygiene ahead of Incident.

Waste findings carry a cost estimate — partial, and bounded by what the data supports. Finding::cost exists and is populated from the AWS Price List API, but only 2 of 25 rules can honestly fill it: idle EC2 instances and idle load balancers, both billed a standing hourly rate. The other five waste rules cannot:

• stopped EC2 instances are not billed for compute at all; the real cost is attached EBS volumes, which the app does not fetch
• both Lambda rules bill GB-seconds times invocations, which memory alone cannot yield
• API Gateway bills per request, and there is no request volume in hand
• orphaned target groups are free

Closing that gap needs usage data (CloudWatch invocation and request metrics, EBS inventory), which is its own piece of work.

Executable pivot — partial. FindingRoute jumps to the relevant service view, but that is a view pivot, not an executable one. The console and CLI actions are scoped to ResourceViews, which excludes Findings, and Finding has no console_url or CLI method of its own. The pieces exist (every model implements DescribableResource); wiring them up means giving the Findings view a way to resolve a finding back to its resource.

Acknowledge/snooze — not started. No state, no persistence, no binding. Finding::key() was designed for it. Worth noting up front: findings without a resource_id have no key, so acknowledging one of those cannot be made to stick, and the UI will need to say so rather than appear to accept it.