🛡️ Sentinel: [HIGH] Fix Cross-Site Request Forgery (CSRF) missing protection

[rauhmaru]

🚨 Severity: HIGH
💡 Vulnerability: Missing Cross-Site Request Forgery (CSRF) protection on forms.
🎯 Impact: Attackers could trick users into executing unwanted actions (e.g., deleting/modifying recipes or items) while authenticated.
🔧 Fix: Initialized Flask-WTF's CSRFProtect, explicitly enabled it in app.py, updated all POST forms in templates to pass the csrf token, disabled it for testing so APIs continue working and appended the learning in sentinel.md.
✅ Verification: python -m pytest passes all tests. Inspecting the forms source code shows <input type="hidden" name="csrf_token" ...>.

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PR created automatically by Jules for task 291809123836521150 started by @rauhmaru

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