docs: Add 1 and update 6 files

SECURITY_REVIEW.md

@@ -44,18 +44,20 @@ This comprehensive security review analyzes the Simple Object Server C++23 appli
 
 ## High-Risk Issues
 
-### 4. **Missing Security Headers**
+### 4. **~~Missing Security Headers~~ [FIXED]**
 - **Location**: HTTP response handling throughout
-- **Risk**: HIGH - Missing standard security headers
-- **Issue**: No implementation of standard security headers
-- **Recommendation**: Add security headers to all responses:
-  ```cpp
-  resp->addHeader("X-Frame-Options", "DENY");
-  resp->addHeader("X-Content-Type-Options", "nosniff");
-  resp->addHeader("X-XSS-Protection", "1; mode=block");
-  resp->addHeader("Strict-Transport-Security", "max-age=31536000");
-  resp->addHeader("Content-Security-Policy", "default-src 'self'");
-  ```
+- **Risk**: ~~HIGH~~ RESOLVED - Security headers now implemented
+- **Fix Implemented**:
+  - Added `add_security_headers()` method in server.cpp
+  - Headers added to all HTTP responses across all endpoints
+  - Implemented headers:
+    - `X-Frame-Options: DENY` - Prevents clickjacking
+    - `X-Content-Type-Options: nosniff` - Prevents MIME sniffing
+    - `X-XSS-Protection: 1; mode=block` - Legacy XSS protection
+    - `Content-Security-Policy` - Restrictive CSP preventing external resources
+    - `Referrer-Policy: strict-origin-when-cross-origin` - Controls referrer info
+    - `Permissions-Policy` - Disables unnecessary browser features
+  - Note: HSTS header commented out by default (requires HTTPS configuration)
 
 ### 5. **Insufficient Input Validation**
 - **Location**: Multiple endpoints (put_handler.cpp, update_handler.cpp)