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/models/mod.rs

@@ -7,3 +7,5 @@ pub mod medication;
 pub mod appointment;
 pub mod share;
 pub mod permission;
+pub mod session;
+pub mod audit_log;