feat: Enable Capacity Config UI (#5164)

- Enables Capacity Config in UI
- Rewrite auto assignment Logic to consider only online agents

fixes: #4990

app/models/concerns/auto_assignment_handler.rb

@@ -0,0 +1,26 @@
+module AutoAssignmentHandler
+  extend ActiveSupport::Concern
+  include Events::Types
+
+  included do
+    after_save :run_auto_assignment
+  end
+
+  private
+
+  def run_auto_assignment
+    # Round robin kicks in on conversation create & update
+    # run it only when conversation status changes to open
+    return unless conversation_status_changed_to_open?
+    return unless should_run_auto_assignment?
+
+    ::AutoAssignment::AgentAssignmentService.new(conversation: self, allowed_agent_ids: inbox.member_ids_with_assignment_capacity).perform
+  end
+
+  def should_run_auto_assignment?
+    return false unless inbox.enable_auto_assignment?
+
+    # run only if assignee is blank or doesn't have access to inbox
+    assignee.blank? || inbox.members.exclude?(assignee)
+  end
+end