EXTRA: Allow anonymous public form submissions

[maebeale]

🤖 suggested review level: 5 Inspect 🔬 nullable FK + person-optional model change with wide nil-person blast radius

Why

Public standalone forms required name + email to build a Person, so a form meant for anonymous feedback couldn't accept a response (it errored with "needs a name and email question").

What

Per-form "Allow anonymous submissions" toggle (admin-set, in the public-link settings).

When on

• Name/email questions stay on the form but become optional — no asterisk, no required attribute, no blank validation.
• A blank-identity submission is recorded with no Person.
• Respondent may still fill name/email to identify themselves (a Person is found/created as before).

Data / model

• form_submissions.person_id is now nullable; FormSubmission belongs_to :person, optional: true.
• Migration is not a backfill — existing rows keep their person_id.

Nil-person handling

• Skips the submitter confirmation email (no address); team FYI still sends, showing "Anonymous respondent".
• Admin submission index/detail show "Anonymous".
• OtherResponses::CaptureFromSubmission no-ops (responses are person-owned).

Seams

• Form#requires_answer? / #optional_identity_field? centralize the identity-optional rule; used by the controller validation and the field partial (new required: local, defaults to field.required so event-registration callers are unchanged).

Notes

• Pre-existing bulk-payment flows already tolerate a nil person via current_user&.person; left untouched (out of scope).