Refactors calendar data management and sync infrastructure

Introduces comprehensive data management system for calendar V2
- Adds IndexedDB storage with pluggable entity services
- Implements EventBus for decoupled event communication
- Creates data seeding mechanism for initial application setup
- Establishes sync and repository abstractions for flexible data handling

src/v2/workers/DataSeeder.ts

@@ -0,0 +1,73 @@
+import { IApiRepository } from '../repositories/IApiRepository';
+import { IEntityService } from '../storage/IEntityService';
+import { ISync } from '../types/CalendarTypes';
+
+/**
+ * DataSeeder - Orchestrates initial data loading from repositories into IndexedDB
+ *
+ * ARCHITECTURE:
+ * - Repository (Mock/Api): Fetches data from source (JSON file or backend API)
+ * - DataSeeder (this class): Orchestrates fetch + save operations
+ * - Service (EventService, etc.): Saves data to IndexedDB
+ *
+ * POLYMORPHIC DESIGN:
+ * - Uses arrays of IEntityService[] and IApiRepository[]
+ * - Matches services with repositories using entityType property
+ * - Open/Closed Principle: Adding new entity requires no code changes here
+ */
+export class DataSeeder {
+  constructor(
+    private services: IEntityService<ISync>[],
+    private repositories: IApiRepository<ISync>[]
+  ) {}
+
+  /**
+   * Seed all entity stores if they are empty
+   */
+  async seedIfEmpty(): Promise<void> {
+    console.log('[DataSeeder] Checking if database needs seeding...');
+
+    try {
+      for (const service of this.services) {
+        const repository = this.repositories.find(repo => repo.entityType === service.entityType);
+
+        if (!repository) {
+          console.warn(`[DataSeeder] No repository found for entity type: ${service.entityType}, skipping`);
+          continue;
+        }
+
+        await this.seedEntity(service.entityType, service, repository);
+      }
+
+      console.log('[DataSeeder] Seeding complete');
+    } catch (error) {
+      console.error('[DataSeeder] Seeding failed:', error);
+      throw error;
+    }
+  }
+
+  private async seedEntity<T extends ISync>(
+    entityType: string,
+    service: IEntityService<T>,
+    repository: IApiRepository<T>
+  ): Promise<void> {
+    const existing = await service.getAll();
+
+    if (existing.length > 0) {
+      console.log(`[DataSeeder] ${entityType} store already has ${existing.length} items, skipping seed`);
+      return;
+    }
+
+    console.log(`[DataSeeder] ${entityType} store is empty, fetching from repository...`);
+
+    const data = await repository.fetchAll();
+
+    console.log(`[DataSeeder] Fetched ${data.length} ${entityType} items, saving to IndexedDB...`);
+
+    for (const entity of data) {
+      await service.save(entity);
+    }
+
+    console.log(`[DataSeeder] ${entityType} seeding complete (${data.length} items saved)`);
+  }
+}