## What
Hardens `CurrencyConversionService` against a slow/unreachable external
FX-rate CDN, which was crashing `/api/cashflow/trend`.
## Sentry
- Fixes PHP-LARAVEL-2X — `RuntimeException: Failed to fetch currency
rates ... cURL error 28: Operation timed out`
- Fixes PHP-LARAVEL-31 — `FatalError: Maximum execution time of 30
seconds exceeded` (Guzzle CurlFactory)
## Root cause
For a historical date the service walked up to 8 lookback dates × 2 URLs
at a **10s timeout each** (~160s worst case → 30s PHP fatal), and a
single network timeout threw a `RuntimeException` that 500'd the whole
request.
## Changes
- **Bounded timeouts**: 3s connect / 5s total per request.
- **Abort on unreachable source**: a connection/timeout error stops the
date walk (after trying the fallback host) instead of repeating the same
timeout for every candidate date. 404s still walk back to earlier
releases.
- **Graceful degradation**: failures return an empty rate map instead of
throwing — conversions already fall back to `0.0` on missing rates, so
the trend endpoint no longer 500s.
- **Persistent cache**: historical releases cached 30d (immutable),
`latest` 6h, unavailable results 10m (dampens retries during an outage).
## Tests
- Updated the former "throws" test to assert graceful degradation to
`0.0`.
- Added: timeout aborts the date walk after 2 attempts; rates cache
across service instances.
- All 12 tests pass.
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