feat: Add PostgreSQL checkpoint storage with pause/resume support

[Kkt04]

Feature Fix Summary

• PostgreSQL Checkpoint Storage: Implemented checkpoint persistence with methods: save_checkpoint, get_checkpoint, delete_checkpoint, list_checkpoints, and cleanup_old_checkpoints.
• Database Migration: Added migration 20260306_0001_add_task_checkpoints.py to create the checkpoint table with proper indexes.
• Pause/Resume Architecture: Centralized pause/resume logic in the Worker base class to remove duplicated implementations.
• Task State Fix: Updated state management:
  • Removed suspended from terminal_states
  • Added it to non_terminal_states
  • Introduced pausable_states
• Worker Pause/Resume Handling: Implemented _handle_pause and _handle_resume methods that save and restore checkpoints.
• URL Validation: Confirmed existing validation in notifications.py using urlparse with scheme and netloc checks.
• Integration Tests: Added 43 tests covering pause/resume behavior, checkpoint persistence, and state transitions.
• Documentation: Created docs/CHECKPOINTS.md and docs/STATE_TRANSITIONS.md.
• Checkpoint Cleanup: Added automatic cleanup in manifest_worker.py when tasks reach completed, failed, or canceled states.