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docs: record the measured reach of the rate lookup

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Ran the real lookup against both live APIs rather than the stubs. Three
findings worth being written down instead of rediscovered:

- Kraken returns ~721 daily candles, so ~2 years for any pair it quotes.
  EUR and GBP both resolve from it directly; the fallback never fires for
  them.
- CoinGecko's free tier answers within 365 days and returns 401
  Unauthorized beyond it — a plan limit wearing an auth error's clothes,
  not a transient failure. The practical ceiling is therefore ~2 years for
  major pairs and 1 year for anything else, which matters given the ask was
  "historical data for up to even a year".
- The two sources disagree by ~2.5% on the same date (2026-05-15: Kraken
  68,047.50 EUR/BTC, CoinGecko 69,743.26) — one exchange's daily close
  versus a cross-exchange average. Neither is wrong, which is the reason
  the ledger records rate_source beside rate rather than presenting a bare
  figure as canonical.

Refusal behaviour confirmed live: an unknown pair and a 2019 date both come
back None rather than falling through to some other number.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jy52j9GRZ6XKa1Zt21LLj
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Padreug 2026-08-31 21:44:21 +02:00
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14
rates.py
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@ -18,6 +18,20 @@ LNbits itself trusts.
addressed by date rather than by series, so it costs one request per date —
fine as a fallback, bad as a default against free-tier rate limits.
Verified coverage (measured against both live APIs, 2026-08-31):
* Kraken returns ~721 daily candles, so **roughly two years** back for any
pair it quotes. EUR and GBP both resolve from it directly.
* CoinGecko's free tier answers within **365 days** and returns `401
Unauthorized` beyond that — a plan limit dressed as an auth error, not a
transient failure. So dates older than ~2 years are unreachable for
everything, and older than a year for currencies Kraken does not quote.
* The two disagree: for 2026-05-15 Kraken gave 68,047.50 EUR/BTC and
CoinGecko 69,743.26, about 2.5% apart — one exchange's daily close versus
a cross-exchange volume-weighted average. Neither is wrong, which is
exactly why the payout records `rate_source` next to `rate` rather than
presenting a bare number as though it were canonical.
Both return None rather than raising. A rate that cannot be established is
a payout that must not happen: the caller records a failed period and lets
a human decide, which is the same thing an underfunded wallet does. The one