Backups to Hetzner S3 fail with 504 Timeout and hang indefinitely (blocking Autovacuum)

### Describe the bug I am using `pgbackweb` to back up a PostgreSQL 15 database to Hetzner S3 Object Storage. The backup process fails during the upload phase with `504 Gateway Timeout` and `400 Bad Request` errors. The db connection is not closed after these errors. It remains active, holding an open transaction slot on the PostgreSQL database. This prevented our Postgres Autovacuum from cleaning up dead tuples until we restarted pgbackweb manually. ### To Reproduce 1. Configure a backup destination using Hetzner S3 (Region: `nbg1`, Endpoint: `https://nbg1.your-objectstorage.com`). 2. Run a backup for a larger database (our backup size is > 5GB) 3. The upload fails with S3 timeout errors (likely due to chunks being too large for the connection/gateway). 4. Check `pg_stat_activity` on the database; the `COPY ... TO stdout` query remains active indefinitely (we observed one running for 17 days). ### Expected behavior If the S3 upload fails, `pgbackweb` should: 1. Retry a reasonable number of times. 2. If retries fail, close the database connection. ### Logs ```text failed to upload file to S3: upload multipart failed, upload id: <ID>, cause: operation error S3: UploadPart, exceeded maximum number of attempts, 3, https response error StatusCode: 504, RequestID: N/A, HostID: N/A, api error GatewayTimeout: The server did not respond in time. failed to upload file to S3: upload multipart failed, upload id: <ID>, cause: operation error S3: UploadPart, https response error StatusCode: 400, RequestID: , HostID: , api error BadRequest: Bad Request ```