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/security/mod.rs

@@ -0,0 +1,7 @@
+pub mod audit_logger;
+pub mod session_manager;
+pub mod account_lockout;
+
+pub use audit_logger::AuditLogger;
+pub use session_manager::SessionManager;
+pub use account_lockout::AccountLockout;