feat: add per-account daily rate limit for outbound emails (#13411)

Introduce a daily cap on non-channel outbound emails to prevent abuse.

Fixes https://linear.app/chatwoot/issue/CW-6418/ses-incident-jan-28

## Type of change

- [x] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality not to work as expected)

## Summary
- Adds a Redis-based daily counter to rate limit outbound emails per
account, preventing email abuse
- Covers continuity emails (WebWidget/API), conversation transcripts,
and agent notifications
  - Email channel replies are excluded (paid feature, not abusable)
- Adds account suspension check in `ConversationReplyMailer` to block
already-queued emails for suspended accounts

  ## Limit Resolution Hierarchy
1. Per-account override (`account.limits['emails']`) — SuperAdmin
configurable
2. Enterprise plan-based (`ACCOUNT_EMAILS_PLAN_LIMITS`
InstallationConfig)
3. Global default (`ACCOUNT_EMAILS_LIMIT` InstallationConfig, default:
100)
  4. Fallback (`ChatwootApp.max_limit` — effectively unlimited)

  ## Enforcement Points
  | Path | Where | Behavior |
  |------|-------|----------|
| WebWidget/API continuity |
`SendEmailNotificationService#should_send_email_notification?` |
Silently skipped |
| Widget transcript | `Widget::ConversationsController#transcript` |
Returns 429 |
| API transcript | `ConversationsController#transcript` | Returns 429 |
| Agent notifications | `Notification::EmailNotificationService#perform`
| Silently skipped |
  | Email channel replies | Not rate limited | Paid feature |
| Suspended accounts | `ConversationReplyMailer` | Blocked at mailer
level |

app/services/messages/send_email_notification_service.rb

@@ -13,6 +13,7 @@ class Messages::SendEmailNotificationService
     return unless Redis::Alfred.set(conversation_mail_key, message.id, nx: true, ex: 1.hour.to_i)
 
     ConversationReplyEmailJob.set(wait: 2.minutes).perform_later(conversation.id, message.id)
+    message.account.increment_email_sent_count
   end
 
   private
@@ -20,6 +21,7 @@ class Messages::SendEmailNotificationService
   def should_send_email_notification?
     return false unless message.email_notifiable_message?
     return false if message.conversation.contact.email.blank?
+    return false unless message.account.within_email_rate_limit?
 
     email_reply_enabled?
   end