Map lg=ah/sh to base locale a/s for the readings-svc /learning call

[mjradwin]

Problem

A PDF download with a daily-learning series and lg=sh (or lg=ah) fails when the fallback fetches from readings-svc:

GET /learning?dsm=on&end=2027-10-31&lg=sh&start=2026-09-01
HTTP/1.1 400 Bad Request
{"error":"Locale 'sh' not found; did you forget to import @hebcal/locales?"}

sh/ah are hebcal-web's own "append the Hebrew name too" spellings, not real @hebcal/locales names. Their base locales are s and a, and the appended Hebrew is drawn here in the renderer (Params.AppendHebrew), not by the sidecar — so the sidecar just needs the base locale.

Fix

learningLocale in internal/repository/readings/learning.go maps ah→a and sh→s at the readings-client boundary — the only code that talks to readings-svc — and passes every other lg through unchanged.

I also audited every other path into the locale packages: all in-process rendering (model/event.go, render.go, /shabbat) routes through model.AliasLocale first, which collapses sh→en / ah→ashkenazi before any locales.LookupTranslation. The /learning sidecar was the only leak, because it needs the short base codes rather than the full locale names.

Test

TestLearningMapsTransliterationLocales asserts ah→a, sh→s, and that he/"" pass through untouched.

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