Add backup retention policy - keep only latest 7 backups on S3

After uploading new backup, delete old backups exceeding 7 files:
- List all .sql.gz files from S3 bucket
- Sort by date descending
- Keep first 7, delete older ones

This prevents unlimited backup accumulation and reduces storage costs.

Author: zhangzhenghao

scripts/backup.sh

@@ -21,6 +21,28 @@ while true; do
     # Remove local SQL file
     rm *.sql.gz
 
+    # Keep only the latest 7 backups on remote
+    BACKUP_FILES=$(s3cmd --access_key=$S3_ACCESS_KEY \
+        --secret_key=$S3_SECRET_KEY \
+        --region=$S3_BUCKET_LOCATION \
+        --host=$S3_HOST_BASE \
+        --host-bucket=$S3_HOST_BUCKET \
+        ls s3://${S3_BUCKET}${S3_PREFIX}/ | grep '\.sql\.gz$' | sort -r)
+    
+    # Count and delete old backups
+    COUNT=0
+    for FILE in $BACKUP_FILES; do
+        COUNT=$((COUNT + 1))
+        if [ $COUNT -gt 7 ]; then
+            s3cmd --access_key=$S3_ACCESS_KEY \
+                --secret_key=$S3_SECRET_KEY \
+                --region=$S3_BUCKET_LOCATION \
+                --host=$S3_HOST_BASE \
+                --host-bucket=$S3_HOST_BUCKET \
+                del $FILE
+        fi
+    done
+
     # Backup 1 day later.
     sleep 86400