Graceful shutdown for in-flight jobs

[codihuston]

Cause: Shutdown may interrupt active tasks without coordination.
Analysis: In-flight tasks can be left in inconsistent states.
Acceptance Criteria:

• Shutdown waits for in-flight tasks or marks them for retry.
• Tasks are not silently lost on shutdown.
  Solution Approach:
• Implement graceful drain with timeout and safe task state transitions.