feat(learn): LanguageTool as a sidecar, reached through the server

compose.languagetool.yaml runs LanguageTool's open-source server
(meyay/languagetool 6.8-13, pinned by digest) on `edge` with no site and
no published port, as user 61081 on a read-only filesystem, without a
log. The server alone reaches it (LANGUAGETOOL_URL): grammar.check is
for those signed in, takes 20,000 characters, 60 checks a minute each
(counted in memory), maps a studyset's language to LanguageTool's code
and keeps nothing. instance.info() names the languages it checks; the
privacy page says what goes there. sidecars/README.md has the steps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/compose.yaml

@@ -113,6 +113,9 @@ services:
       # The Jellyfin server suggested to people; empty: jellyfin. on the
       # instance's own domain, if one answers there; `none`: none.
       JELLYFIN_URL: ${JELLYFIN_URL:-}
+      # The LanguageTool server checking spelling and grammar, reached by the server
+      # alone; empty: none (compose.languagetool.yaml sets its own).
+      LANGUAGETOOL_URL: ${LANGUAGETOOL_URL:-}
       # Where people's data and the backups are, which the app shows: `EU`, a
       # country (`DE`) or a subdivision (`US-CA`); empty: it says nothing.
       DATA_LOCATION: ${DATA_LOCATION:-}