fix(viewer): keep completed Team migration terminal (#1560)

## Description

Make completed legacy Team migration terminal in Viewer routes. Canonical completed Teams suppress superseding legacy drafts, stale direct reads return 404, and every mutation/refresh path fails closed. Coordinator roster reads also use bounded retry and cached fallback behavior.

Live verification confirmed the completed SRE Team is absent from the migration summary and its stale detail URL returns 404.

## Type of Change

- [ ] 🚀 Feature (new functionality)
- [x] 🐛 Bug fix (fixes an issue)
- [ ] 📚 Documentation (docs-only change)
- [ ] 🔧 Maintenance (refactor, chore, CI, etc.)
- [ ] 🧪 Testing (test-only changes)

## Testing

- [x] Relevant checks pass locally (`pnpm run tsc`, `pnpm run lint`, `pnpm run test`)
- [x] Added/updated tests for changes
- [x] Manually verified changes work as expected

## Checklist

- [x] Code follows project style (`pnpm run lint` passes for touched files)
- [x] Self-review completed
- [x] Documentation updated (if needed)
- [x] No new warnings introduced