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/audit_logger.rs

@@ -0,0 +1,34 @@
+use anyhow::Result;
+use mongodb::bson::oid::ObjectId;
+use crate::models::audit_log::{AuditLog, AuditLogRepository, AuditEventType};
+
+#[derive(Clone)]
+pub struct AuditLogger {
+    repository: AuditLogRepository,
+}
+
+impl AuditLogger {
+    pub fn new(db: &mongodb::Database) -> Self {
+        Self {
+            repository: AuditLogRepository::new(db),
+        }
+    }
+
+    pub async fn log_event(
+        &self,
+        event_type: AuditEventType,
+        user_id: Option<ObjectId>,
+        email: Option<String>,
+        ip_address: String,
+        resource_type: Option<String>,
+        resource_id: Option<String>,
+    ) -> Result<ObjectId> {
+        self.repository
+            .log(event_type, user_id, email, ip_address, resource_type, resource_id)
+            .await
+    }
+
+    pub async fn get_user_audit_logs(&self, user_id: &ObjectId) -> Result<Vec<AuditLog>> {
+        self.repository.find_by_user(user_id).await
+    }
+}