Add security headers middleware and tests; improve user model schema

- Added security headers middleware to enforce best practices (e.g., XSS and clickjacking prevention, CSP, HSTS in production).
- Updated `User` model schema: refined field constraints and switched `preferences` to `JSONB` for PostgreSQL compatibility.
- Introduced tests to validate security headers across endpoints and error responses.
- Ensured headers like `X-Frame-Options`, `X-Content-Type-Options`, and `Permissions-Policy` are correctly configured.

backend/tests/api/test_security_headers.py

@@ -0,0 +1,94 @@
+# tests/api/test_security_headers.py
+import pytest
+from fastapi.testclient import TestClient
+from unittest.mock import patch
+
+from app.main import app
+
+
+@pytest.fixture
+def client():
+    """Create a FastAPI test client for the main app."""
+    # Mock get_db to avoid database connection issues
+    with patch("app.main.get_db") as mock_get_db:
+        def mock_session_generator():
+            from unittest.mock import MagicMock
+            mock_session = MagicMock()
+            mock_session.execute.return_value = None
+            mock_session.close.return_value = None
+            yield mock_session
+
+        mock_get_db.side_effect = lambda: mock_session_generator()
+        yield TestClient(app)
+
+
+class TestSecurityHeaders:
+    """Tests for security headers middleware"""
+
+    def test_x_frame_options_header(self, client):
+        """Test that X-Frame-Options header is set to DENY"""
+        response = client.get("/health")
+        assert "X-Frame-Options" in response.headers
+        assert response.headers["X-Frame-Options"] == "DENY"
+
+    def test_x_content_type_options_header(self, client):
+        """Test that X-Content-Type-Options header is set to nosniff"""
+        response = client.get("/health")
+        assert "X-Content-Type-Options" in response.headers
+        assert response.headers["X-Content-Type-Options"] == "nosniff"
+
+    def test_x_xss_protection_header(self, client):
+        """Test that X-XSS-Protection header is set"""
+        response = client.get("/health")
+        assert "X-XSS-Protection" in response.headers
+        assert response.headers["X-XSS-Protection"] == "1; mode=block"
+
+    def test_content_security_policy_header(self, client):
+        """Test that Content-Security-Policy header is set"""
+        response = client.get("/health")
+        assert "Content-Security-Policy" in response.headers
+        assert "default-src 'self'" in response.headers["Content-Security-Policy"]
+        assert "frame-ancestors 'none'" in response.headers["Content-Security-Policy"]
+
+    def test_permissions_policy_header(self, client):
+        """Test that Permissions-Policy header is set"""
+        response = client.get("/health")
+        assert "Permissions-Policy" in response.headers
+        assert "geolocation=()" in response.headers["Permissions-Policy"]
+        assert "microphone=()" in response.headers["Permissions-Policy"]
+        assert "camera=()" in response.headers["Permissions-Policy"]
+
+    def test_referrer_policy_header(self, client):
+        """Test that Referrer-Policy header is set"""
+        response = client.get("/health")
+        assert "Referrer-Policy" in response.headers
+        assert response.headers["Referrer-Policy"] == "strict-origin-when-cross-origin"
+
+    def test_strict_transport_security_not_in_development(self, client):
+        """Test that Strict-Transport-Security header is not set in development"""
+        from app.core.config import settings
+
+        # In development, HSTS should not be present
+        if settings.ENVIRONMENT == "development":
+            response = client.get("/health")
+            assert "Strict-Transport-Security" not in response.headers
+
+    def test_security_headers_on_all_endpoints(self, client):
+        """Test that security headers are present on all endpoints"""
+        # Test health endpoint
+        response = client.get("/health")
+        assert "X-Frame-Options" in response.headers
+        assert "X-Content-Type-Options" in response.headers
+
+        # Test root endpoint
+        response = client.get("/")
+        assert "X-Frame-Options" in response.headers
+        assert "X-Content-Type-Options" in response.headers
+
+    def test_security_headers_on_404(self, client):
+        """Test that security headers are present even on 404 responses"""
+        response = client.get("/nonexistent-endpoint")
+        assert response.status_code == 404
+        assert "X-Frame-Options" in response.headers
+        assert "X-Content-Type-Options" in response.headers
+        assert "X-XSS-Protection" in response.headers