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/sidecars/README.md

@@ -18,6 +18,9 @@ site changes.
 | Sidecar | Compose file | Site |
 |---|---|---|
 | Jellyfin, which people watch and listen to learn from | `compose.jellyfin.yaml` | `jellyfin.caddy` |
+| LanguageTool, which checks spelling and grammar | `compose.languagetool.yaml` | none |
+
+Both together: `COMPOSE_FILE=compose.yaml:compose.jellyfin.yaml:compose.languagetool.yaml`.
 
 ## Jellyfin
 
@@ -96,3 +99,53 @@ Steps, in order; ✱ marks a step as root.
 
 Switching it off: the compose file out of `COMPOSE_FILE`, then `docker compose up -d
 --remove-orphans`; `deploy/jellyfin/` stays until removed by hand.
+
+## LanguageTool
+
+LanguageTool's open-source server beside the instance (ADR 0024), which checks spelling
+and grammar for Halkyon Learn: Check in the assistant's field, and the underlines while
+writing (the app's Settings › Writing). Only the instance's server reaches it
+(`LANGUAGETOOL_URL`, set by the compose file): LanguageTool has no sign-in of its own,
+so it gets no site, and the server checks who asks and how often (signed in, 60 checks a
+minute each). Nothing written is kept or logged, LanguageTool's own log included: Docker
+keeps none for it. It checks without n-gram data (LanguageTool's statistical checks of
+words like their and there, tens of GB per language), in about 1.5 GB of memory.
+
+Steps, in order; ✱ marks a step as root.
+
+1. ✱ **Its user and folder** (the operator, on the instance's server, as a user with
+   sudo): the user and group ID 61081 are LanguageTool's alone, as the compose file runs
+   it (`useradd` warns that the ID lies above the system range, which is meant: it must
+   not meet a container's IDs).
+
+   ```sh
+   sudo groupadd --system --gid 61081 languagetool
+   sudo useradd --system --uid 61081 --gid 61081 --no-create-home --shell /usr/sbin/nologin languagetool
+   cd ~deploy/halkyon-learn/deploy
+   sudo mkdir -p languagetool/fasttext
+   sudo chown -R languagetool:languagetool languagetool
+   ```
+
+2. **Switch it on** (the operator, on the instance's server, as `deploy`): in
+   `deploy/.env`, `COMPOSE_FILE=compose.yaml:compose.languagetool.yaml`.
+
+3. **Start it** (the operator, on the instance's server, as `deploy`):
+   `cd ~/halkyon-learn/deploy && docker compose up -d`. The server is recreated with it
+   (its `LANGUAGETOOL_URL`); the instance is away for seconds. At its first start
+   LanguageTool fetches fastText's model of languages (`lid.176.bin`, 126 MB, from
+   Facebook's `dl.fbaipublicfiles.com`) into `deploy/languagetool/fasttext/`, which it
+   uses to tell which language a note is in; checks start once it is there.
+
+4. **Check it**: on the instance's server, as `deploy`, `docker compose ps languagetool`
+   says `healthy` after a minute; then, from anywhere, the instance names the languages
+   it checks:
+
+   ```sh
+   curl -s -X POST -H 'Content-Type: application/json' -d '{}' https://<domain>/instance/info
+   ```
+
+   answers with `grammarLanguages`, `de-DE` and `en-US` among them.
+
+Switching it off: the compose file out of `COMPOSE_FILE`, then `docker compose up -d
+--remove-orphans`; the app's Writing settings then say the instance checks nothing, and
+`deploy/languagetool/` stays until removed by hand.