Missing user-agent header causing 403 records in off_chain_vote_fetch_error

**OS**
Your OS:Ubuntu 22.04

**Versions**
The `db-sync` version (eg `cardano-db-sync --version`): 13.6.0.4
PostgreSQL version: 16.4

**Build/Install Method**
The method you use to build or install `cardano-db-sync`: GitHub binary

**Run method**
The method you used to run `cardano-db-sync` (eg Nix/Docker/systemd/none): systemd

**Additional context**
when db-sync is fetching metadata without user-agent in the header of the outbound request, response is 403 errors which populate the off_chain_vote_fetch_error table for instances where the metadata maybe hosted on a dedicated domain or behind a firewall

**Problem Report**
id | voting_anchor_id | fetch_error | fetch_time | retry_count
-------+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------+-------------
43932 | 744 | Error Offchain Voting Anchor: HTTP Response error from https://cardanofoundation.org/governance/drepMetaData.jsonld resulted in HTTP status code : 403 "Forbidden" | 2025-04-07 18:02:58.451774 | 60