fix: address review feedback for backup infrastructure

- compose.yaml: rewrite db-backup to use heredoc script instead of inline
cron command, fixing date escaping and adding temp-file pipeline for
reliable error detection
- compose.yaml: fix pipefail issue by writing pg_dump to tmp file before
compression and rotation
- restore.sh: pass PGPASSWORD explicitly via docker compose exec -e
- restore.sh: use ". .env" with set -a/+a instead of fragile xargs export

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

compose.yaml

@@ -35,7 +35,17 @@ services:
     entrypoint: ["sh", "-c"]
     command:
       - |
-        echo "0 3 * * * pg_dump -h db -U gmrelay -d gmrelay_db | gzip > /backups/gmrelay_db_\$$(date +\%Y\%m\%d_\%H\%M\%S).sql.gz && find /backups -name 'gmrelay_db_*.sql.gz' -type f -mtime +\$${BACKUP_RETENTION_DAYS} -delete" | crontab -
+        cat > /usr/local/bin/backup.sh << 'EOF'
+        #!/bin/sh
+        set -e
+        TMPFILE="/tmp/backup_$$.sql"
+        pg_dump -h db -U gmrelay -d gmrelay_db > "$TMPFILE"
+        gzip "$TMPFILE"
+        mv "$TMPFILE.gz" "/backups/gmrelay_db_$(date +%Y%m%d_%H%M%S).sql.gz"
+        find /backups -name 'gmrelay_db_*.sql.gz' -type f -mtime +${BACKUP_RETENTION_DAYS} -delete
+        EOF
+        chmod +x /usr/local/bin/backup.sh
+        echo "0 3 * * * /usr/local/bin/backup.sh" | crontab -
         crond -f
 
   bot: