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/models/account.rb

@@ -29,6 +29,7 @@ class Account < ApplicationRecord
   include Featurable
   include CacheKeys
   include CaptainFeaturable
+  include AccountEmailRateLimitable
 
   SETTINGS_PARAMS_SCHEMA = {
     'type': 'object',

app/models/concerns/account_email_rate_limitable.rb

@@ -0,0 +1,49 @@
+module AccountEmailRateLimitable
+  extend ActiveSupport::Concern
+
+  OUTBOUND_EMAIL_TTL = 25.hours.to_i
+  EMAIL_LIMIT_CONFIG_KEY = 'ACCOUNT_EMAILS_LIMIT'.freeze
+
+  def email_rate_limit
+    account_limit || global_limit || default_limit
+  end
+
+  def emails_sent_today
+    Redis::Alfred.get(email_count_cache_key).to_i
+  end
+
+  def within_email_rate_limit?
+    return true if emails_sent_today < email_rate_limit
+
+    Rails.logger.warn("Account #{id} reached daily email rate limit of #{email_rate_limit}. Sent: #{emails_sent_today}")
+    false
+  end
+
+  def increment_email_sent_count
+    Redis::Alfred.incr(email_count_cache_key).tap do |count|
+      Redis::Alfred.expire(email_count_cache_key, OUTBOUND_EMAIL_TTL) if count == 1
+    end
+  end
+
+  private
+
+  def email_count_cache_key
+    @email_count_cache_key ||= format(
+      Redis::Alfred::ACCOUNT_OUTBOUND_EMAIL_COUNT_KEY,
+      account_id: id,
+      date: Time.zone.today.to_s
+    )
+  end
+
+  def account_limit
+    self[:limits]&.dig('emails')&.to_i
+  end
+
+  def global_limit
+    GlobalConfig.get(EMAIL_LIMIT_CONFIG_KEY)[EMAIL_LIMIT_CONFIG_KEY]&.to_i
+  end
+
+  def default_limit
+    ChatwootApp.max_limit.to_i
+  end
+end