feat(reshape): surface staleness banner when Cost Explorer cache is old

[cristim]

Summary

• Backend: getReshapeRecommendations now reads GetRecommendationsFreshness and populates recs_staleness ("" / "soft" / "hard") and recs_collected_at on the response; freshness failures are non-fatal (logged, staleness fields omitted)
• Frontend: loadReshapeRecommendations calls the new renderReshapeStalenessBanner which inserts a banner slot above the recommendations table using warning/error CSS classes with a relative-time age label
• Tests: classifyRecsAge unit tests pin the 12h/24h thresholds; frontend banner snapshot tests cover all three staleness states

Closes #150

Test plan

• go test ./internal/api/... passes
• cd frontend && npx jest riexchange passes (37 tests)
• cd frontend && npx tsc --noEmit passes
• Dev env: pause the scheduler, wait 12h, open RI Exchange tab, verify soft banner appears

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 8ba597b7-52ce-4040-a53c-e366b5f671aa

📥 Commits

Reviewing files that changed from the base of the PR and between bc7bf0f and a027e4d.

📒 Files selected for processing (7)

• frontend/src/__tests__/riexchange-permissions.test.ts
• frontend/src/__tests__/riexchange.test.ts
• frontend/src/api/index.ts
• frontend/src/api/riexchange.ts
• frontend/src/riexchange.ts
• internal/api/handler_ri_exchange.go
• internal/api/handler_ri_exchange_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/issue-150-staleness-banner

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