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/db/mongodb_impl.rs

@@ -116,6 +116,12 @@ impl MongoDb {
         }
     }
     
+    /// Get a reference to the underlying MongoDB Database
+    /// This is needed for security services in Phase 2.6
+    pub fn get_database(&self) -> Database {
+        self.database.clone()
+    }
+    
     // ===== User Methods =====
     
     pub async fn create_user(&self, user: &User) -> Result<Option<ObjectId>> {