Dynamic scheduled tasks

[cupatea]

This pull request closes [#186] and brings in a resque-scheduler style dynamic schedules feature, allowing you to add or remove recurring tasks at runtime without touching your static config file.

What’s new:

• Dynamic scope (static: false) on SolidQueue::RecurringTask model.
• Renamed methods/vars in SolidQueue::Scheduler::RecurringSchedule to distinguish static vs. dynamic schedules.
• On boot, @configured_tasks now includes both static and dynamic tasks.
• Added SolidQueue::Scheduler::RecurringSchedule.update_scheduled_tasks:
  • Schedules any new dynamic tasks added since the last run.
  • Unschedules any dynamic tasks removed from the database.
  • Refreshes @configured_tasks to match the current DB state.
• SolidQueue::Configuration no longer requires a non-blank static config file - pure dynamic scheduling is now supported.
• SolidQueue::Scheduler watches for changes after launch and updates its metadata so the running process always reflects the true set of recurring tasks.

Tests verify that adding or dropping dynamic tasks at runtime correctly updates what’s scheduled.