BITMAG-1248 Replace java.util.Timer with ScheduledExecutorService + virtual threads

[Knud-Aage]

java.util.Timer runs every scheduled task on a single shared thread, and an uncaught exception in any TimerTask silently kills that thread — every other job scheduled on it stops firing with no error surfaced. This replaces all Timer/TimerTask usage with a new
ScheduledVirtualThreadExecutor.
ScheduledVirtualThreadExecutor (bitrepository-core) pairs a single-thread ScheduledExecutorService "ticker" — which only decides when to fire — with a virtual-thread-per-task executor that actually runs each firing. This means:

• a slow or blocking task run can't delay the next scheduled tick
• an uncaught exception in one task run is logged and only kills that one virtual thread, not the scheduler
• tasks no longer compete for a small bounded thread pool
  Converted call sites (TimerTask → Runnable, Timer → ScheduledVirtualThreadExecutor):
• TimerBasedScheduler (workflow scheduling)
• AuditTrailCollector / LocalAuditTrailPreserver
• CollectionBasedConversationMediator (conversation cleanup)
• StatusCollector (monitoring service)
  JobScheduler gained a shutdown() method, threaded through WorkflowManager.shutdown() and called from IntegrityServiceManager and Pillar.close(), so the scheduler's executors are actually released on shutdown instead of leaking threads.

Added ScheduledVirtualThreadExecutorTest and TimerBasedSchedulerTest to cover the new scheduling behavior.