fixing Hardcoded Non-Cryptographic Secret and change the port to 8080 .

- version : 2.6
- Details : adding .env file that has the secret key and use it .. using
  different python modules and removing the unused route `/add_lesson`.
//############################################//
	modified:   .gitignore
	modified:   app.py
	modified:   requirements.txt
	modified:   serve.sh
//############################################//

requirements.txt

@@ -1,2 +1,3 @@
 Flask==3.1.1
 gunicorn==23.0.0
+python-dotenv==1.1.1