refactor: prefork child hardening and app.py documentation

[pratyush618]

Summary

• Prefork stdout flush — flush stdout before breaking on shutdown message to prevent partial result loss
• app.py docstring — document the mixin composition pattern (4 mixins + base class) so new contributors understand the file's structure

Test plan

• ruff clean, mypy 0 errors
• No behavioral changes — doc + defensive flush only