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>

.gitignore

@@ -28,6 +28,7 @@ deploy/.env
 deploy/assistants.env
 # A sidecar's files on the server, such as Jellyfin's configuration and media
 deploy/jellyfin/
+deploy/languagetool/
 # Dictionary files an instance loads: made by halkyon_learn_curator, not kept in git.
 deploy/dictionaries/*.jsonl
 deploy/dictionaries/*.jsonl.gz