Add comprehensive email validation for blocked users

[akshayutture-augment]

Test 3

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Replicated from ai-code-review-evaluation/discourse-cursor#3

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Note

Adds server-side blocked email validation with whitelist/blacklist support, updates signup to return structured errors, and prevents reusing rejected emails client-side.

• Backend:
  • Email validation: Introduces EmailValidator applying SiteSetting whitelist/blacklist and new BlockedEmail.should_block? check; User now uses validates :email, email: true.
  • Blocked emails: Adds BlockedEmail model (tracks match_count, last_match_at, action_type) with migration create_blocked_emails.
  • Signup errors: UsersController#create now returns errors and values in JSON on failure.
  • I18n: Adds user.email.blocked message.
• Frontend:
  • CreateAccountController: tracks rejectedEmails to invalidate previously failed accountEmail; emailValidation observes rejectedEmails.
• Tests:
  • Adds specs for EmailValidator and BlockedEmail, plus blocked_email fabricator.

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[akshayutture-augment]

cursor review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!

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