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/storage/IStore.ts

@@ -0,0 +1,18 @@
+/**
+ * IStore - Interface for IndexedDB ObjectStore definitions
+ *
+ * Each entity store implements this interface to define its schema.
+ * Enables Open/Closed Principle: IndexedDBContext works with any IStore.
+ */
+export interface IStore {
+  /**
+   * The name of the ObjectStore in IndexedDB
+   */
+  readonly storeName: string;
+
+  /**
+   * Create the ObjectStore with its schema (indexes, keyPath, etc.)
+   * Called during database upgrade (onupgradeneeded event)
+   */
+  create(db: IDBDatabase): void;
+}