feat(cli): add event loop factory option - #660

Closes #659.

Summary

• add --loop-factory module:callable to the worker and scheduler CLI
• resolve and validate the configured factory before creating an event loop
• resolve worker factories inside each worker subprocess
• let an explicit worker factory override automatic uvloop selection
• preserve existing behavior when no factory is configured
• document the option for both commands

Motivation

Some integrations require a specific event loop implementation. For example, async Psycopg on Windows requires a selector-based loop instead of the default ProactorEventLoop.

Applications can currently influence TaskIQ-created loops through process-wide asyncio policies, but those policies are deprecated in Python 3.14 and scheduled for removal in Python 3.16.

This provides a generic application-controlled mechanism and a forward-compatible alternative for the Windows compatibility case discussed in #641, without changing TaskIQ's default loop on any platform.

Compatibility

The scheduler uses AnyIO's loop_factory backend option so the feature remains compatible with TaskIQ's Python 3.10 minimum. The worker keeps its existing loop lifecycle and changes only how the loop is selected.