Replace single retentionDays with a three-tier BackupRetentionPolicy:
- Daily: keep all backups (presets: 3, 7, 14 days; default 7)
- Weekly: keep one per calendar week (presets: 1, 2, 4 weeks; default 4)
- Monthly: keep one per calendar month (presets: 1, 3, 6 months; default 1)
Pruning sorts backups newest-first and applies each tier's cutoff,
keeping only the newest entry per ISO week/month bucket. The Instance
Settings General page now shows three preset selectors (no icon, matches
existing page design). Remove Database icon import.
Compress database backups with gzip (.sql.gz), reducing file size ~83%.
Add backup retention configuration to Instance Settings UI with preset
options (7 days, 2 weeks, 1 month). The backup scheduler now reads
retention from the database on each tick so changes take effect without
restart. Default retention changed from 30 to 7 days.
