Implement job scheduler

[FionaVerzivolli]

Create job scheduler with stats and checkpoints so that we have a background thread pool for concurrent job execution, that has a priority based job queueing system and job dependency management and scheduling policies.

GPT PR summary:

• Multi-threaded job execution: Worker thread pool processes jobs concurrently without blocking main database operations, supporting configurable thread counts for optimal resource utilization

• Priority-based job queue: Four-tier priority system (CRITICAL, HIGH, NORMAL, LOW) ensures critical maintenance tasks like checkpoints execute before routine operations, maintaining system responsiveness under load

• Automatic checkpoint management: Time-based and WAL-size triggers automatically flush dirty pages and create recovery points, preventing unbounded log growth while maintaining data durability

• Recurring job scheduling: Background tasks like health checks, version pruning, and WAL cleanup run at configurable intervals, providing continuous system maintenance without manual intervention

• Job lifecycle tracking: Complete job state management (PENDING → RUNNING → COMPLETED/FAILED) with timeout handling, retry capabilities, and detailed execution statistics for monitoring system health

• Health monitoring integration: Real-time success rate tracking and system health indicators support 99.98% uptime requirements through proactive failure detection and automated recovery mechanisms