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/controllers/api/v1/accounts/conversations_controller.rb

@@ -70,8 +70,10 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
 
   def transcript
     render json: { error: 'email param missing' }, status: :unprocessable_entity and return if params[:email].blank?
+    return head :too_many_requests unless @conversation.account.within_email_rate_limit?
 
     ConversationReplyMailer.with(account: @conversation.account).conversation_transcript(@conversation, params[:email])&.deliver_later
+    @conversation.account.increment_email_sent_count
     head :ok
   end
 

app/controllers/api/v1/widget/conversations_controller.rb

@@ -35,12 +35,9 @@ class Api::V1::Widget::ConversationsController < Api::V1::Widget::BaseController
   end
 
   def transcript
-    if conversation.present? && conversation.contact.present? && conversation.contact.email.present?
-      ConversationReplyMailer.with(account: conversation.account).conversation_transcript(
-        conversation,
-        conversation.contact.email
-      )&.deliver_later
-    end
+    return head :too_many_requests unless conversation.present? && conversation.account.within_email_rate_limit?
+
+    send_transcript_email
     head :ok
   end
 
@@ -77,6 +74,16 @@ class Api::V1::Widget::ConversationsController < Api::V1::Widget::BaseController
 
   private
 
+  def send_transcript_email
+    return if conversation.contact&.email.blank?
+
+    ConversationReplyMailer.with(account: conversation.account).conversation_transcript(
+      conversation,
+      conversation.contact.email
+    )&.deliver_later
+    conversation.account.increment_email_sent_count
+  end
+
   def trigger_typing_event(event)
     Rails.configuration.dispatcher.dispatch(event, Time.zone.now, conversation: conversation, user: @contact)
   end