Deploy: LanguageTool spell checker in production, with optional n-grams

- docker-compose.yml: a `languagetool` service (erikvl87/languagetool:6.8),
  internal only, 1 GB heap, one CPU, a 28 GB memory limit that is room for the
  n-gram page cache (reclaimable, not memory in use), and a low-priority
  warm-up read of the n-gram files at start so first checks hit RAM even on
  mechanical disks. `init` creates $DATA_ROOT/languagetool-ngrams; the app
  containers get LANGUAGETOOL_URL (empty turns the check off).
- `webmail.sh ngrams [langs]` (default en de fr es): deploy/ngrams.py, run in
  the backend image, downloads LanguageTool's n-gram data with resume, checks
  free space first (unpacked is ~2x the zip: ~26 GB for all four), unpacks into
  a temp folder and moves it into place, then recreates the engine to load it.
  languagetool.org refuses Python's default User-Agent, so it sends its own.
- `doctor` reports whether the spell checker answers and which n-gram
  languages are installed.
- docs/nas-install.md and .env.prod.example describe all of it.

Verified locally: German n-grams (1.58 GB -> 3.06 GB) load, and catch what the
rules alone do not ("liegt ihm Schrank" -> im, "Der Patenten" -> Patienten).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker/.env.prod.example

@@ -59,6 +59,18 @@ RSPAMD_MEM_LIMIT=256m
 RSPAMD_REDIS_MEM_LIMIT=160m
 RSPAMD_REDIS_MAXMEMORY=128mb
 
+# --- Spelling and grammar check (LanguageTool) -------------------------------
+# The compose window's spell checker. Leave empty to turn it off for everyone;
+# unset uses the bundled container (http://languagetool:8010).
+#LANGUAGETOOL_URL=
+# Java heap. 1g covers a household or small team.
+LANGUAGETOOL_HEAP=1g
+# Mostly file cache for the optional n-gram data (`webmail.sh ngrams`, ~26 GB
+# unpacked for en/de/fr/es), which the kernel gives back under memory pressure.
+# Without n-grams it never comes near this. Lower it on a small NAS.
+LANGUAGETOOL_MEM_LIMIT=28g
+LANGUAGETOOL_CPUS=1.0
+
 # --- Search -----------------------------------------------------------------
 # pg   = Postgres full-text. No extra container, no extra RAM. Default.
 # meili = Meilisearch: typo tolerance, better ranking, faster on large mailboxes.