Normalize mixed Gmail paragraph breaks

[parterburn]

Summary

• normalize Gmail edge <br> separators into standalone paragraph breaks
• preserve one consistent blank line across mixed Gmail HTML representations
• cover trailing, leading, explicit, empty-block, NBSP, duplicate, repeated, nested, and inline separator forms
• verify the full paid-user inbound path stores normalized HTML
• verify the entry page renders the exact div/br sequence and four text lines

Verification

• bundle exec rspec spec/models/email_processor_spec.rb spec/features/entries_spec.rb — 39 examples, 0 failures
• bundle exec rspec — 236 examples, 0 failures, 1 expected pending