feat(backup): enhance postgres_backup.sh robustness and retention logic

[jorsol]

• Implement exponential backoff retry logic (3 retries) for wal-g backup-push.
• Modernize retention policy using jq-based JSON filtering for safer deletion.
• Improve safety with strict recovery-state checks and exit code handling.
• Fix potential crash by initializing defaults for retention variables.

[zalando-robot]

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