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 |

spec/services/messages/send_email_notification_service_spec.rb

@@ -99,6 +99,20 @@ describe Messages::SendEmailNotificationService do
         end
       end
 
+      context 'when account email rate limit is exceeded' do
+        let(:inbox) { create(:inbox, account: account, channel: create(:channel_widget, account: account, continuity_via_email: true)) }
+        let(:conversation) { create(:conversation, account: account, inbox: inbox) }
+
+        before do
+          conversation.contact.update!(email: 'test@example.com')
+          allow_any_instance_of(Account).to receive(:within_email_rate_limit?).and_return(false) # rubocop:disable RSpec/AnyInstance
+        end
+
+        it 'does not enqueue job' do
+          expect { service.perform }.not_to have_enqueued_job(ConversationReplyEmailJob)
+        end
+      end
+
       context 'when channel does not support email notifications' do
         let(:inbox) { create(:inbox, account: account, channel: create(:channel_sms, account: account)) }
         let(:conversation) { create(:conversation, account: account, inbox: inbox) }