Add outbound HTTP proxy support for backend requests

[dafal]

Summary

• make RomM's shared backend aiohttp and httpx clients honor standard outbound proxy environment variables
• route the HLTB bootstrap HTTP calls through the same proxy-aware client helpers and add focused tests for proxy env detection
• document supported HTTP_PROXY/HTTPS_PROXY/NO_PROXY variables, including lowercase variants, in the Docker Compose example

[gantoine]

can you give some concrete examples?

[zurdi15]

I think this is for services that runs behind a proxy or a restricted net that won't allow requests against 3rd party sites unless you configure the backend to go through the proxy?

[dafal]

I run romM connected on internal docker network without direct internet access.