fix: make server JSON parsing and proxy handling robust

[ThanhNguyxn]

Summary

• replace strict Content-Type == application/json check with request.is_json and validate parsed payload shape
• stop overriding http_proxy / https_proxy at module import time so network-constrained environments keep working
• lazily import Pipeline and load_config to avoid heavy import-time dependency coupling in server module
• add unit tests for JSON content-type with charset and proxy environment preservation

Why

The server currently rejects valid requests that send application/json; charset=utf-8, and it also clears proxy env vars globally on import. Together these behaviors can break real deployments (client compatibility + model/download connectivity).

Testing

• python -m pytest glmocr/tests/test_unit.py -k \"server_accepts_json_content_type_with_charset or server_module_does_not_override_proxy_env\"