build(train): Train's server and BRouter deployable beside the instance (ADR 0030, D46)

deploy/compose.train.yaml adds Train's server at train.halkyon.eu, a
member of Learn's register, with its database halkyon_train in the
instance's PostgreSQL, and BRouter beside it alone (v1.7.10, checked by
SHA-256, no log), its segments of the operator's region kept current
monthly. Nothing changes until deploy/.env names the compose file:
redeploy.sh and backup.sh take Train in only then, and the operator's
steps are in sidecars/README.md. restic now forgets snapshots grouped by
host alone, so adding Train's dump leaves no group behind for good.

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/.env.example

@@ -37,13 +37,35 @@ DOMAIN=http://localhost
 
 # Sidecars switched on beside the instance (sidecars/README.md), each by its compose file
 # after the instance's own: compose.jellyfin.yaml, a Jellyfin people watch and listen to
-# learn from. Unset: the instance alone.
+# learn from; compose.languagetool.yaml, spelling and grammar checked; compose.train.yaml,
+# Halkyon Train's server with its BRouter. Unset: the instance alone.
 #COMPOSE_FILE=compose.yaml:compose.jellyfin.yaml
 
 # With the Jellyfin sidecar: the domain it answers at, which the instance then suggests
 # to its people (the app's Jellyfin sign-in fills it in).
 #JELLYFIN_DOMAIN=jellyfin.example.org
 
+# With Halkyon Train beside the instance (compose.train.yaml, sidecars/README.md,
+# Train): the domain its apps reach it at, its database's password, its own session
+# pepper (never change it once people sign in), INSTANCE_SECRET above set, and the
+# region of BRouter's segments, `west,south,east,north` in whole degrees, multiples of
+# 5 (empty: Europe's, -25,35,40,70, about 3.3 GB). Generate each secret:
+# openssl rand -hex 32. Its server is told of accounts gone without INSTANCE_MEMBERS;
+# setting it, name Train's too (`http://train-server:8082/`).
+#TRAIN_DOMAIN=train.example.org
+#TRAIN_POSTGRES_PASSWORD=
+#TRAIN_SESSION_KEY_HASH_PEPPER=
+#BROUTER_REGION=
+# The Loop's firmware through the instance (Train's ADR 0030, D40): `off` to stop
+# relaying it; empty: on.
+#LOOP_FIRMWARE_RELAY=
+# Train's map (ADR 0030, D22): a MapLibre style over HTTPS, with its name, credit and
+# page; empty: OpenFreeMap's.
+#MAP_STYLE_URL=
+#MAP_SOURCE_NAME=
+#MAP_ATTRIBUTION=
+#MAP_ABOUT_URL=
+
 # Postgres password, shared by the postgres and server services.
 # Generate: openssl rand -base64 32
 POSTGRES_PASSWORD=