fix: return a friendly 422 instead of a 500 when a form fails validation

[jontyms]

Summary

Fixes GlitchTip ONBOARD-3N (7 events). post_form called model(**inp) without catching pydantic's ValidationError, so any submission that failed form validation returned a 500 and the member saw nothing useful. Two causes seen in the events:

• A malformed email (e.g. no TLD) on the sign-up form.
• An admin opened the compliance training before fix: make the reporter scenario's correct answer unambiguous #422's new wording was deployed and submitted afterwards. The old option text is no longer a valid choice.

Now it's a 422 that names the fields that failed, using the labels the member sees, which form.js shows in its error banner:

Please check: Preferred Email. If the form changed since you opened it, reload the page and try again.

• app/util/forms.py: new form_field_labels() maps field keys to their visible label. Quiz radios use their scenario heading.
• app/routes/api.py: catches ValidationError, logs a warning with field names and error types only (the input is PII), and raises the 422.

Testing

• New tests cover both cases from the GlitchTip events (a stale quiz option and a malformed email).
• pytest: 65 passed, 1 skipped. ruff and pyrefly clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01VJTrZnhWMDKQ51nmshVqsJ