reprompter

<role>
Senior full-stack engineer specializing in real-time dashboard systems,
REST API integration, and cache-aware data pipelines.
</role>

<context>
- Frontend + backend + tests required (cross-layer scope)
- Data source: CoinGecko API (free tier, 30 calls/min rate limit)
- Existing API behavior must be preserved
</context>

<task>
Deliver a real-time crypto dashboard backed by a cache-aware CoinGecko proxy API,
with unit tests for both API and UI, without breaking existing API contracts.
</task>

<requirements>
- Build dashboard UI with loading, error, empty, and stale-data states
- Implement backend CoinGecko proxy with JSON schema validation + configurable cache TTL
- Preserve backward compatibility for all existing API consumers
- Add deterministic unit tests for frontend rendering states and backend edge cases
- Cache must serve stale data on upstream failure (stale-while-revalidate pattern)
</requirements>

<constraints>
- No direct client-side calls to CoinGecko (all traffic through proxy)
- No breaking changes to existing API response fields or status codes
- Mock all external network boundaries in tests
- Rate limit CoinGecko calls to stay within free tier (30/min)
</constraints>

<output_format>
- Backend: /api/prices endpoint returning { prices: [...], cached: bool, updatedAt: ISO }
- Frontend: React component with 5s auto-refresh interval
- Tests: Vitest suite with >=80% branch coverage
</output_format>

<success_criteria>
- Dashboard auto-updates every 5s and shows "stale" indicator when cache is old
- Proxy returns normalized data within 200ms (cache hit) / 2s (cache miss)
- Existing API integration tests still pass with zero modifications
</success_criteria>