Python Server SDK: docstring pass B, relay, skills, livewire, search, prefabs, and remaining modules

[hey-august]

Repo: signalwire-python. Files: relay/call.py, relay/client.py, relay/event.py, core/skill_base.py, skills/*/skill.py, livewire/__init__.py, search/*, pom/*, utils/*, web/*, prefabs/*.

• Same standard as pass A, porting from Fern pages/reference/python/{relay,agents}/. Add module docstrings where missing (rest 47%).

Done when: hand-written coverage ≥ 90%; lint clean.

Part of #724