Add more security and audit

backend/requirements.txt

@@ -1,12 +1,14 @@
-flask==3.0.3
+flask==3.1.1
 flask-login==0.6.3
 flask-sqlalchemy==3.1.1
-flask-migrate==4.0.7
-flask-cors==4.0.1
-psycopg2-binary==2.9.9
-gunicorn==22.0.0
-python-dotenv==1.0.1
-werkzeug==3.0.3
-sqlalchemy==2.0.31
-authlib==1.3.1        # OAuth2 (Microsoft + Google, later)
+flask-migrate==4.1.0
+flask-limiter==3.9.0        # rate limiting
+flask-talisman==1.1.0       # security headers (CSP, HSTS, X-Frame-Options, ...)
+psycopg2-binary==2.9.10
+gunicorn==23.0.0
+python-dotenv==1.1.0
+werkzeug==3.1.3
+sqlalchemy==2.0.41
+authlib==1.4.1
 requests==2.32.3
+redis==5.2.1                # backend voor flask-limiter in productie