[codex] Fix backup concurrency CI test

[chuanman2707]

Summary

• keep the backup concurrency/collision test timestamp inside the active retention window
• compute the expected backup filename stem from the runtime timestamp instead of a stale fixed date

Why

CI failed on main after #169 because the test used 2026-04-18; on 2026-05-19 that manual backup is older than the 30-day retention window, so pruning correctly removed the base file and kept only the collision-suffix safety-floor file.

Validation

• cargo test --manifest-path mhm/src-tauri/Cargo.toml backup::runner::tests::run_backup_once_serializes_concurrent_backups -- --nocapture
• cargo test --manifest-path mhm/src-tauri/Cargo.toml
• cargo check (mhm/src-tauri)
• cargo clippy --all-targets -- -D warnings (mhm/src-tauri)
• rustfmt --edition 2024 --check mhm/src-tauri/src/backup/runner.rs