docs: record the measured reach of the rate lookup

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

docs/operations.md

@@ -67,6 +67,23 @@ exchange providers: those are spot tickers with no date parameter, and the
 rate history behind the admin chart is RAM-only, single-currency and wiped
 on restart.
 
+How far back this actually reaches, measured against both live APIs:
+
+| payday age | major pair (EUR, GBP, USD…) | currency Kraken doesn't quote |
+|---|---|---|
+| under 1 year | Kraken | CoinGecko |
+| 1–2 years | Kraken | **unavailable** |
+| over 2 years | **unavailable** | **unavailable** |
+
+CoinGecko's free tier returns `401 Unauthorized` past 365 days — a plan
+limit wearing an auth error's clothes. Unavailable means the period fails
+and says so; it never guesses.
+
+The two sources disagree by a couple of percent on the same date (for
+2026-05-15: Kraken 68,047.50 EUR/BTC, CoinGecko 69,743.26 — one exchange's
+close versus a cross-exchange average). That is why the ledger records
+`rate_source` beside `rate` instead of presenting a bare figure as canonical.
+
 Where payroll converts, it hands `create_invoice` a **sat** amount, because
 `create_invoice` always prices fiat itself and cannot be told a rate. The
 rate used and its source are recorded on the payout row either way — in

rates.py

@@ -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