fix(backup): back up the postgres database (was unprotected)

The immich recipe declared backupbot.backup only on the app service (whose volumes
are all excluded), so the postgres database — all of immich users metadata, albums,
people, settings — was NOT backed up at all. Restoring a backup yielded an empty DB.

Add a database-service postgres backup mirroring the coop-cloud convention
(matrix-synapse, keycloak, ...): a /pg_backup.sh config-mounted hook driven by
backupbot pre/restore hooks. backup = pg_dump | gzip -> backup.sql in the postgres
volume; restore = terminate immich-server connections, FORCE-drop, recreate, reimport
(the VectorChord/pgvecto.rs extensions and all data round-trip cleanly).

abra.sh

@@ -0,0 +1 @@
+export PG_BACKUP_VERSION=v1