Add database backup/restore script with auto-pruning

scripts/db-backup.sh provides three commands:
- backup: creates a timestamped, gzipped pg_dump (custom format) in ./backups/
- restore: drops, recreates, and loads a backup with confirmation prompt
- list: shows available backups with sizes and dates

Supports --keep N flag for automatic pruning of backups older than N days,
making it cron-friendly for daily automated backups. Also adds backups/
and *.dump.gz to .gitignore.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -24,6 +24,11 @@ postgres_data/
 redis_data/
 pgdata/
 
+# Database backups
+backups/
+*.dump
+*.dump.gz
+
 # SSL
 letsencrypt/