Adds idempotency guard to prevent race conditions when multiple users
submit commands for the same play simultaneously.
Changes:
- Add PlayLockedException for locked play detection
- Implement lock check in checks_log_interaction()
- Acquire lock (play.locked = True) before processing commands
- Release lock (play.locked = False) after play completion
- Add warning logs for rejected duplicate submissions
- Add /diagnostics endpoint to health server for debugging
This prevents database corruption and duplicate processing when users
spam commands like "log xcheck" while the first is still processing.
Tested successfully in Discord - duplicate commands now properly return
PlayLockedException with instructions to wait.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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