infra: add PostgreSQL daily backup via pg_dump with rotation

- Add db-backup service to compose.yaml (postgres:17-alpine + cron)
- Add pgbackups volume for backup storage
- Add scripts/restore.sh for manual restore from latest backup
- Update .env.example with BACKUP_RETENTION_DAYS and BACKUP_VOLUME_NAME
- Document backup/restore flow in README

Bump version -> 1.15.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.ServiceDefaults/packages.lock.json

@@ -66,6 +66,12 @@
           "OpenTelemetry.Api": "[1.15.3, 2.0.0)"
         }
       },
+      "SecurityCodeScan.VS2019": {
+        "type": "Direct",
+        "requested": "[5.6.7, )",
+        "resolved": "5.6.7",
+        "contentHash": "WIE9RJswdSc2j+rLz2gW6U+gMUjMHzY2j7C/CL8/R2olXNM/+twarfMnWqm+rZodDBvaYDApJyxM8mVYf9FGrQ=="
+      },
       "Microsoft.Extensions.AmbientMetadata.Application": {
         "type": "Transitive",
         "resolved": "10.2.0",