## Problem
`PHP-LARAVEL-2M` (High, escalating, 38 events) — `RuntimeException:
Failed to fetch currency rates for xxx on 2025-12-31: HTTP 404` crashing
`/dashboard`.
A user holds an account in currency `xxx` (ISO 4217 "no/unknown
currency"). No rate file exists for it, so every candidate URL returns
404. `CurrencyConversionService::fetchRates` threw an unhandled
`RuntimeException` that bubbled past the graceful-degradation logic in
`ExchangeRateService::convert` and broke the whole dashboard render.
## Fix
Distinguish **permanent** 404s from **transient** failures:
- All sources return 404 → log a warning and return `[]`. Callers
already handle empty rates (return unconverted / zero amount).
- Timeouts / 5xx / connection errors → still throw, so real outages stay
visible.
## Tests
- New: unknown base currency with all-404 sources returns `0.0` instead
of throwing.
- Existing "throws when both primary and fallback fail" (500s) still
passes — transient errors still surface.
Fixes PHP-LARAVEL-2M.
bef657c2ed