Form submission statistics (per-form + cross-form aggregate)

🤖 From Claude: Filed while spiking the post-event survey feature.

What's wanted

A statistics view for a Form's submissions — aggregate the answers across all FormSubmissions of that form:

• Per selectable question (radio/dropdown/checkbox, incl. Likert scales): distribution/counts per option.
• Free-text questions: list/browse responses.
• Response rate / submission count over time.

Cross-form

Allow selecting/filtering multiple forms to see combined, cross-form statistics — e.g. compare Day 1 vs Day 2 survey Likert results side by side, or roll up several events' surveys.

Why

The post-event survey brings evaluations in-house (previously an external tool). The value is in reading the aggregate results, not just individual submissions. Likert questions especially need distribution charts.

Notes

• Data is already there: FormSubmission + FormAnswer (answers stored verbatim, plus question_name_when_answered).
• Consider the dataviz guidance for the charts.

Start by reading the FormSubmission and FormAnswer models and the dataviz guidance mentioned in the issue. Define how per-form and cross-form aggregates, free-text browsing, response counts over time, and Likert distributions should be presented. Done means the statistics view supports the requested single- and multi-form comparisons with appropriate charts.