Third fallback URL for fiat currencies when both jsDelivr and pages.dev fail #161

The README asks every client to add a fallback (#90), but both URLs today are served from the same npm publish: if a release is late or broken, both fail together (as in #142 for crypto).

I work on AllRatesToday. We now serve a URL-compatible mirror of the /v1/currencies endpoints from the ECB reference table, keyless and free:

https://allratestoday.com/api/compat/currency-api/latest/v1/currencies/eur.json
https://allratestoday.com/api/compat/currency-api/2026-10-15/v1/currencies/usd.json
https://allratestoday.com/api/compat/currency-api/latest/v1/currencies.json

Same JSON keys and lowercase codes, so the existing pseudo-code in #90 just gets one more line. Limits, stated plainly: about 30 fiat currencies, euro-area business days, no crypto or metals. Anything not covered returns 404, so a client falls through instead of getting a wrong number.

Would you accept a short "third-party fallback" note under the Cloudflare fallback section? A PR with the two lines follows. If you would rather not list an outside domain in the README, I understand; the endpoint stays up either way for people who find it here.