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/backups.md

@@ -196,6 +196,21 @@ the way back from a restore that went wrong.
    docker compose up -d
    ```
 
+Train's database, where `.env` switches Train on (`compose.train.yaml`, Halkyon Train's
+ADR 0030, D46), is in the same snapshots as `latest-train.dump`, and is restored the
+same way, its server stopped meanwhile (`docker compose stop train-server`), from the
+same snapshot as Learn's, so that the register's accounts and Train's people agree:
+
+```sh
+restic dump <snapshot> /home/deploy/backups/latest-train.dump >~/backups/restore-train.dump
+docker compose exec -T postgres \
+  pg_restore -U halkyon_learn -d postgres --clean --create <~/backups/restore-train.dump
+```
+
+`pg_restore --create` makes the database anew owned as it was, by `halkyon_train`,
+whose role is the cluster's and so survives the restore (on a new server, make it first:
+`sidecars/README.md`, *Train*, step 2).
+
 To check a backup without touching the instance, restore it into a database of its own
 next to the real one, look, and drop it: