feat: add FinOps model choice opportunity scoring

[ccf]

Summary

• score workflow-level model downshift opportunities inside FinOps cost modeling
• surface potential savings and confidence in the FinOps modeling tab
• true up the roadmap status for model-choice opportunity scoring

Testing

• PYTHONPATH=src pytest tests/test_finops.py -q
• ruff check src/primer/common/schemas.py src/primer/server/services/finops_service.py tests/test_finops.py
• cd frontend && ./node_modules/.bin/vitest run --run src/components/finops/tests/workflow-cost-table.test.tsx src/components/finops/tests/workflow-compare-card.test.tsx src/components/finops/tests/model-choice-opportunity-table.test.tsx
• cd frontend && ./node_modules/.bin/eslint src/components/finops/model-choice-opportunity-table.tsx src/components/finops/modeling-tab.tsx src/components/finops/tests/model-choice-opportunity-table.test.tsx src/types/api.ts
• cd frontend && ./node_modules/.bin/tsc --noEmit

---

Note

Medium Risk
Adds new backend analytics and schema fields to the cost-modeling response plus a new UI surface, which could affect query performance and the correctness of cost/success calculations used for recommendations.

Overview
Adds model-choice opportunity scoring to FinOps cost modeling: the backend now analyzes sessions by workflow archetype to suggest cheaper models that maintain similar success rates, returning top opportunities plus aggregated potential monthly savings in CostModelingResponse.

Surfaces these recommendations in the FinOps modeling tab via a new ModelChoiceOpportunityTable component (with tests), and updates API types/schemas accordingly; also marks the roadmap item as shipped.

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