feat: scale email rate limits by agent count for paid plans (#13966)

Paid plan accounts with many agents were hitting the flat daily email
rate limit cap.
This multiplies the plan base limit by the account's agent seat count,
so larger teams
get proportionally higher limits. Free/hacker plan keeps the flat limit
unchanged.

Fixes https://linear.app/chatwoot/issue/CW-6664

## How to test
1. Set up `ACCOUNT_EMAILS_PLAN_LIMITS` config with plan limits (e.g.,
`{"hacker": 10, "startups": 20, "business": 30, "enterprise": 40}`)
2. Create an account on a paid plan (e.g., startups) with multiple agent
seats
3. Verify `account.email_rate_limit` returns `base_limit × agent_seats`
(e.g., 20 × 5 = 100)
4. Create an account on the hacker plan — verify limit stays flat (10)
5. Set a per-account override via super admin `limits.emails` — verify
it takes priority over the multiplied limit

## What changed
- `plan_email_limit` now multiplies the base plan limit by agent seat
count for paid plans
- Added `free_plan?` helper to skip the multiplier for the default/free
plan

enterprise/app/models/enterprise/account/plan_usage_and_limits.rb

@@ -75,6 +75,14 @@ module Enterprise::Account::PlanUsageAndLimits # rubocop:disable Metrics/ModuleL
   end
 
   def plan_email_limit
+    base_limit = plan_base_email_limit
+    return nil if base_limit.nil?
+    return base_limit if free_plan?
+
+    base_limit * [agent_limits.to_i, 1].max
+  end
+
+  def plan_base_email_limit
     config = InstallationConfig.find_by(name: 'ACCOUNT_EMAILS_PLAN_LIMITS')&.value
     return nil if config.blank? || plan_name.blank?
 
@@ -84,6 +92,11 @@ module Enterprise::Account::PlanUsageAndLimits # rubocop:disable Metrics/ModuleL
     nil
   end
 
+  def free_plan?
+    default_plan = InstallationConfig.find_by(name: 'CHATWOOT_CLOUD_PLANS')&.value&.first
+    default_plan.present? && plan_name&.downcase == default_plan['name']&.downcase
+  end
+
   def default_captain_limits
     max_limits = { documents: ChatwootApp.max_limit, responses: ChatwootApp.max_limit }.with_indifferent_access
     zero_limits = { documents: 0, responses: 0 }.with_indifferent_access