Enhance security architecture and guidelines across documentation and middleware; implement input validation, logging improvements, and security headers in API handlers.

docs/SECURITY.md

@@ -98,6 +98,17 @@ This document describes the security architecture, configurations, and best prac
   - `INTERNAL` (500)
 - **No Secrets in Logs**: Passwords and tokens are never logged
 
+### Security Headers
+
+The application sets comprehensive security headers:
+
+- **X-Content-Type-Options**: `nosniff` - Prevents MIME type sniffing
+- **X-Frame-Options**: `DENY` - Prevents clickjacking (except for WOPI endpoints)
+- **X-XSS-Protection**: `1; mode=block` - Enables XSS filtering
+- **Content-Security-Policy**: Restrictive policy allowing only necessary sources
+- **Referrer-Policy**: `strict-origin-when-cross-origin` - Controls referrer information
+- **CORS**: Restricted to allowed origins with credentials support
+
 ## Network Security
 
 ### TLS Configuration