Corrections: record who made each correction log entry

[iabaako]

Stack position

• Order: 8 of 13
• Branch: feat/321-correction-log-user, from feat/318-backcheck-target-metrics
• PR base: feat/318-backcheck-target-metrics (retarget to main once Backchecks: track progress against the backcheck target #318 merges)
• Blocked by: Backchecks: track progress against the backcheck target #318
• Next: Backchecks: attribute mismatches to an error source and report an adjusted error rate #301 branches from this branch

Summary

The correction log records when, why and from where a change was made, but not
who made it
(corrections.py:236-249).
Add a user field to every log entry. The backcheck attribution log in #301
uses the same identity.

Scope

1. Reviewer identity.
   • Default to the OS login (getpass.getuser()).
   • Add a "Reviewer name" setting that overrides the default and is remembered
     across sessions. It holds a display name only, never credentials.
   • Expose one helper, for example get_reviewer_name(), used by every
     logging path.
2. user column in corr_log_{alias}.
   • The shared correction form and apply_correction_entries fill it on
     every entry, from any source.
   • Backfill existing logs with an empty value, extending the existing
     column backfill.
3. Correction Log display shows the User column.
   correction_log.csv in the replication package includes it.

Acceptance criteria

• New entries from every source record the reviewer.
• The "Reviewer name" setting overrides the OS login and persists.
• Existing logs load with no data loss.
• Tests cover the default, the override and the backfill.