fix: Safely propagate JoinErrors in core/users.rs

[Cylae]

Remediates unsafe spawn_blocking termination in verify_async inside server_manager/src/core/users.rs. Replaced .unwrap_or_else which hid standard JoinErrors with map_err() to explicitly report join faults in log before returning None. Updated audit log in CHANGES-2026-09-13.md accordingly. Verification passed all ./verify.sh batteries.

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PR created automatically by Jules for task 12095885245616627147 started by @Cylae

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