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/handlers/mod.rs

@@ -1,25 +1,14 @@
 pub mod auth;
 pub mod health;
-pub mod users;
-pub mod shares;
 pub mod permissions;
+pub mod shares;
+pub mod users;
+pub mod sessions;
 
-// Auth handlers
-pub use auth::{
-    register, login, recover_password,
-};
-
-// User handlers
-pub use users::{
-    get_profile, update_profile, delete_account,
-    get_settings, update_settings, change_password,
-};
-
-// Health handlers
+// Re-export commonly used handler functions
+pub use auth::{register, login, recover_password};
 pub use health::{health_check, ready_check};
-
-// Share handlers
-pub use shares::{create_share, list_shares, get_share, update_share, delete_share};
-
-// Permission handlers
+pub use shares::{create_share, list_shares, update_share, delete_share};
 pub use permissions::check_permission;
+pub use users::{get_profile, update_profile, delete_account, change_password, get_settings, update_settings};
+pub use sessions::{get_sessions, revoke_session, revoke_all_sessions};