Fixes#1175
Preserves the forwarded host and port when Shelfmark builds OIDC
callback URLs. The reverse-proxy examples now retain custom ports as
well.
Tests:
- `uv run pytest -n 0 -q tests/core/test_proxy_headers.py
tests/core/test_oidc_routes.py`
- `uv run ruff check shelfmark/main.py tests/core/test_proxy_headers.py`
- `uv run ruff format --check shelfmark/main.py
tests/core/test_proxy_headers.py`
