Prevent stale submitted project count

Add lock to prevent stale reads in the case this runs concurrently for the same project

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: zetter-rpf

app/models/lesson.rb

@@ -41,8 +41,10 @@ def submitted_count
   end
 
   def recalculate_submitted_projects_count!
-    count = school_projects.in_state(:submitted).count
-    update!(submitted_projects_count: count)
+    with_lock do
+      count = school_projects.in_state(:submitted).count
+      update!(submitted_projects_count: count)
+    end
   end
 
   private