Add grounded AI state explainer

[danielmcauley]

Summary

• add a cached AI-powered state explainer route with a grounded fallback summary
• render the explainer on state pages with mode switching for overview, affordability, and economy
• add Prisma storage, shared types, env docs, and tests for fallback generation and client behavior

Validation

• npm run db:push
• npm test
• npm run lint
• npm run build

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fetch mode-specific brief even when OpenAI is disabled

The effect exits immediately when aiEnabled is false, so selecting “affordability” or “economy” never loads mode-specific content. Because currentBrief falls back to the initial overview brief, the UI can show a non-overview button as active while still rendering overview text/citations. This breaks the advertised mode switching behavior.

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