Add Meilisearch index to automated GCS backup

[monneyboi]

Currently PostgreSQL is backed up daily to GCS via poliloom-backup.timer, but Meilisearch is only backed up manually via Makefile targets (index-dump, index-snapshot) to local dumps/ directory.

Task

Add Meilisearch dump to the automated backup pipeline so it's also uploaded to GCS.

Options

1. Extend poliloom-backup.service to also dump Meilisearch
2. Create separate poliloom-backup-index.service/timer

Reference

• Existing backup: systemd-timers/poliloom-backup.service
• Meilisearch dump logic: Makefile (index-dump target)