run migrations automatically on backend startup (AUTO_MIGRATIONS)

Add a migrate.sh config (ported from lasuite-docs) that the backend
entrypoint runs when AUTO_MIGRATIONS=true (default). It loads secrets,
waits for the DB, and idempotently applies pending migrations via
'migrate --check' then 'migrate --noinput'. The manual 'abra app cmd
backend migrate' now delegates to the same script.

abra.sh

@@ -4,6 +4,7 @@ export ABRA_ENTRYPOINT_VERSION=v11
 export NGINX_CONF_VERSION=v6
 export ONLYOFFICE_CONF_VERSION=v2
 export PG_BACKUP_VERSION=v4
+export MIGRATE_VERSION=v1
 
 environment() {
   # this exports all the secrets as environment variables
@@ -11,8 +12,7 @@ environment() {
 }
 
 migrate() {
-  environment
-  python manage.py migrate --noinput
+  /migrate.sh
 }
 
 trigger_wopi() {