fix: handle permission denied on backup directory

The backups directory may have been created by Docker/root, making it
unwritable by the deploy user. The script now detects this and attempts
to fix permissions automatically (chmod, then sudo chown as fallback).

Also added shared/db/backups to .gitignore.

.gitignore

@@ -3,6 +3,7 @@ node_modules
 docker-compose.override.yml
 shared/db-logs
 shared/db/data
+shared/db/backups
 shared/db/loga
 .cursor
 # dependencies (bun install)