Backfill display name for anonymous accounts created during #589 window

[mircealungu]

Context

PR #589 (merged 2026-04-29) removed the uuid from anonymous account display names but inadvertently left name = NULL. PR #591 fixes the going-forward path by deriving name from the generated username (adjective + animal → "Quirky Owl").

Anonymous accounts created between the merge of #589 and the merge of #591 still have name = NULL in the database. They will continue to:

• crash the teacher-classroom management page,
• show a broken Congratulations screen at the end of an exercise session.

See zeeguu/web#1095 for the user-visible symptoms.

Scope

• Affected rows: anonymous users (User.is_anonymous == True, or whatever the equivalent check is) where name IS NULL, created between 2026-04-29 19:53 UTC and the merge time of Fix display names and add avatar color randomization for anonymous accounts #591.
• Expected count: small (single hours / low single days of signups). Worth confirming with a SELECT COUNT(*) before running the backfill.

Suggested approach

One-shot script (or SQL) that, for each affected user, derives a display name from the existing username:

"quirky_owl42" -> "Quirky Owl"

i.e. split on _, strip trailing digits from the second segment, capitalize each part, join with a space. The username column is already populated for these users (the bug was only in name), so no need to regenerate.

Acceptance

• All previously NULL anonymous-account name values are populated.
• Teacher classroom page renders without crashing for affected users.
• Congratulations page renders a sensible name for affected users.

[klnyzzz33]

See the comments in #591 for the backfill script.