feat: complete Phase 2.6 - Security Hardening

- Implement session management with device tracking
- Implement audit logging system
- Implement account lockout for brute-force protection
- Add security headers middleware
- Add rate limiting middleware (stub)
- Integrate security services into main application

Build Status: Compiles successfully
Phase: 2.6 of 8 (75% complete)

backend/src/middleware/security_headers.rs

@@ -0,0 +1,39 @@
+use axum::{
+    extract::Request,
+    http::HeaderValue,
+    middleware::Next,
+    response::Response,
+};
+
+pub async fn security_headers_middleware(
+    req: Request,
+    next: Next,
+) -> Response {
+    let mut response = next.run(req).await;
+
+    let headers = response.headers_mut();
+    
+    // Security headers
+    headers.insert(
+        "X-Content-Type-Options",
+        HeaderValue::from_static("nosniff"),
+    );
+    headers.insert(
+        "X-Frame-Options",
+        HeaderValue::from_static("DENY"),
+    );
+    headers.insert(
+        "X-XSS-Protection",
+        HeaderValue::from_static("1; mode=block"),
+    );
+    headers.insert(
+        "Strict-Transport-Security",
+        HeaderValue::from_static("max-age=31536000; includeSubDomains"),
+    );
+    headers.insert(
+        "Content-Security-Policy",
+        HeaderValue::from_static("default-src 'self'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'"),
+    );
+
+    response
+}