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).

compose.yml

@@ -67,6 +67,21 @@ services:
       - postgres:/var/lib/postgresql/data
     networks:
       - backend
+    deploy:
+      labels:
+        backupbot.backup: "${ENABLE_BACKUPS:-true}"
+        backupbot.backup.pre-hook: "/pg_backup.sh backup"
+        backupbot.backup.volumes.postgres.path: "backup.sql"
+        backupbot.restore.post-hook: "/pg_backup.sh restore"
+    configs:
+      - source: pg_backup
+        target: /pg_backup.sh
+        mode: 0555
+
+configs:
+  pg_backup:
+    name: ${STACK_NAME}_pg_backup_${PG_BACKUP_VERSION}
+    file: pg_backup.sh
 
 secrets:
   db_password: