Remove unused blob preload from submit_form attachments_index

Bullet was warning that ActiveStorage::Attachment => [:blob] was eager
loaded but never used (Bullet doesn't detect blob access through to_json
method delegation). Remove the preload; blob lazy-loads per attachment
on demand, which is acceptable for the small attachment counts in signing forms.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/controllers/submit_form_controller.rb

@@ -115,7 +115,7 @@ class SubmitFormController < ApplicationController
 
   def build_attachments_index(submission)
     ActiveStorage::Attachment.where(record: submission.submitters, name: :attachments)
-                             .preload(:blob).index_by(&:uuid)
+                             .index_by(&:uuid)
   end
 
   def require_email_2fa?(submitter)