Refactor entity services with hybrid sync pattern

Introduces BaseEntityService and SyncPlugin to eliminate code duplication across entity services

Improves:
- Code reusability through inheritance and composition
- Sync infrastructure for all entity types
- Polymorphic sync status management
- Reduced boilerplate code by ~75%

Supports generic sync for Event, Booking, Customer, and Resource entities

src/storage/IEntityService.ts

@@ -0,0 +1,46 @@
+import { ISync, EntityType, SyncStatus } from '../types/CalendarTypes';
+
+/**
+ * IEntityService<T> - Generic interface for entity services with sync capabilities
+ *
+ * All entity services (Event, Booking, Customer, Resource) implement this interface
+ * to enable polymorphic sync status management in SyncManager.
+ *
+ * ENCAPSULATION: Services encapsulate sync status manipulation.
+ * SyncManager does NOT directly manipulate entity.syncStatus - it delegates to the service.
+ *
+ * POLYMORFI: SyncManager works with Array<IEntityService<any>> and uses
+ * entityType property for runtime routing, avoiding switch statements.
+ */
+export interface IEntityService<T extends ISync> {
+  /**
+   * Entity type discriminator for runtime routing
+   * Must match EntityType values: 'Event', 'Booking', 'Customer', 'Resource'
+   */
+  readonly entityType: EntityType;
+
+  /**
+   * Mark entity as successfully synced with backend
+   * Sets syncStatus = 'synced' and persists to IndexedDB
+   *
+   * @param id - Entity ID
+   */
+  markAsSynced(id: string): Promise<void>;
+
+  /**
+   * Mark entity as sync error (max retries exceeded)
+   * Sets syncStatus = 'error' and persists to IndexedDB
+   *
+   * @param id - Entity ID
+   */
+  markAsError(id: string): Promise<void>;
+
+  /**
+   * Get current sync status for an entity
+   * Used by SyncManager to check entity state
+   *
+   * @param id - Entity ID
+   * @returns SyncStatus or null if entity not found
+   */
+  getSyncStatus(id: string): Promise<SyncStatus | null>;
+}