Add automatic crash diagnostics and session log retention

[thomas9120]

Summary

• Add early, idempotent diagnostics initialization for both server entrypoints.
• Capture stderr, Python exceptions, worker failures, and native fault tracebacks in timestamped logs.
• Preserve console output and continue startup when logging is unavailable.
• Retain the 10 newest diagnostic logs and document troubleshooting guidance.

Testing

• Added comprehensive diagnostics tests covering startup failures, exception capture, console behavior, retention, idempotency, filesystem failures, and native crashes.
• Backend test coverage updated to document the new suite.