scheduler: split into modules for readability

- mod.rs: Actor struct and message handlers
- executor.rs: Job execution logic
- backoff.rs: Backoff calculation and jitter

Also removed redundant running_jobs field (use job_handles instead).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: aster-void