Adds audit logging and sync management infrastructure

Introduces comprehensive audit trail system with:
- AuditService to track entity changes
- SyncManager for background sync of audit entries
- New CoreEvents for entity and audit tracking
- Simplified sync architecture with event-driven approach

Prepares system for enhanced compliance and change tracking

src/repositories/MockAuditRepository.ts

@@ -0,0 +1,47 @@
+import { IApiRepository } from './IApiRepository';
+import { IAuditEntry } from '../types/AuditTypes';
+import { EntityType } from '../types/CalendarTypes';
+
+/**
+ * MockAuditRepository - Mock API repository for audit entries
+ *
+ * In production, this would send audit entries to the backend.
+ * For development/testing, it just logs the operations.
+ */
+export class MockAuditRepository implements IApiRepository<IAuditEntry> {
+  readonly entityType: EntityType = 'Audit';
+
+  async sendCreate(entity: IAuditEntry): Promise<void> {
+    // Simulate API call delay
+    await new Promise(resolve => setTimeout(resolve, 100));
+
+    console.log('MockAuditRepository: Audit entry synced to backend:', {
+      id: entity.id,
+      entityType: entity.entityType,
+      entityId: entity.entityId,
+      operation: entity.operation,
+      timestamp: new Date(entity.timestamp).toISOString()
+    });
+  }
+
+  async sendUpdate(_id: string, _entity: IAuditEntry): Promise<void> {
+    // Audit entries are immutable - updates should not happen
+    throw new Error('Audit entries cannot be updated');
+  }
+
+  async sendDelete(_id: string): Promise<void> {
+    // Audit entries should never be deleted
+    throw new Error('Audit entries cannot be deleted');
+  }
+
+  async fetchAll(): Promise<IAuditEntry[]> {
+    // For now, return empty array - audit entries are local-first
+    // In production, this could fetch audit history from backend
+    return [];
+  }
+
+  async fetchById(_id: string): Promise<IAuditEntry | null> {
+    // For now, return null - audit entries are local-first
+    return null;
+  }
+}