feat(service): configure gateway proxy read timeout

[mikemikimike]

Summary

• Make the gateway proxy_read_timeout configurable per service.
• Preserve the existing 300-second default and add a server-level default override.
• Render the configured timeout for both normal HTTP and WebSocket proxy locations.

Validation

• uv run pytest src/tests -q — 356 passed, 4891 skipped on Windows (POSIX-only tests skipped by repository configuration)
• uv run ruff check .
• uv run ruff format --check .
• uv run pyright -p .
• git diff --check

Closes #4239