Adjust logs pertinence

[pfongkye]

Drop orphan Review rows when Slack channel is archived

When a Slack channel is archived, every chat.update / chat.postMessage from a review-flow webhook returns is_archived and the Review row keeps pointing at the dead channel — re-triggering the failure (and commit 1's webhook slack work failed log) on every subsequent MR update, note or push. Channels do get archived as a routine cleanup; treat it as an expected operational state rather than a hard error.

Demote expired_trigger_id to structured warn

errorMiddleware already replies with EXPIRED_TRIGGER_ID_ERROR_MESSAGE when Slack rejects a views.open with expired_trigger_id, but the underlying error was still logged through logger.error(error) as a bare stack trace.

Demote users_not_found to info and unify email lookup

"Failed to to fetch slack user with emails" was noisy in the logs: it covered the perfectly normal case of a GitLab user who has left the company (or is external, or has no corporate Slack account) and emitted it at error level with no structured context.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.29%. Comparing base (9314be3) to head (efa1c4a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   89.13%   89.29%   +0.16%     
==========================================
  Files         103      104       +1     
  Lines        2254     2269      +15     
  Branches      435      436       +1     
==========================================
+ Hits         2009     2026      +17     
+ Misses        232      230       -2     
  Partials       13       13              

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