**feat(tests, docs, storage): add statistics storage tests and enhance documentation**

- **Tests**: Added new `statistics-storage.test.ts` to validate statistics storage functionality across scenarios including saving, retrieving, time-based partitioning, and backward compatibility. Ensured tests dynamically handle missing environment variables by skipping S3-related tests when credentials are unavailable.
- **Docs**: Enhanced `statistics.md` with detailed explanations of scalability improvements, including adaptive flush timing, batched updates, and time-based partitioning. Improved readability and structure.
- **Storage**: Updated all storage adapters to integrate time-based partitioning and maintain backward compatibility with legacy statistics storage formats.
- **Dependencies**: Added `dotenv` to support environmental variable management for storage adapter tests.

**Purpose**: Strengthen system reliability by adding comprehensive test coverage for statistics storage, improve scalability documentation, and ensure consistency across storage adapters with robust implementations.

src/storage/adapters/memoryStorage.ts

@@ -328,6 +328,7 @@ export class MemoryStorage extends BaseStorage {
    * @param statistics The statistics data to save
    */
   protected async saveStatisticsData(statistics: StatisticsData): Promise<void> {
+    // For memory storage, we just need to store the statistics in memory
     // Create a deep copy to avoid reference issues
     this.statistics = {
       nounCount: {...statistics.nounCount},
@@ -336,6 +337,9 @@ export class MemoryStorage extends BaseStorage {
       hnswIndexSize: statistics.hnswIndexSize,
       lastUpdated: statistics.lastUpdated
     }
+    
+    // Since this is in-memory, there's no need for time-based partitioning
+    // or legacy file handling
   }
 
   /**
@@ -355,5 +359,8 @@ export class MemoryStorage extends BaseStorage {
       hnswIndexSize: this.statistics.hnswIndexSize,
       lastUpdated: this.statistics.lastUpdated
     }
+    
+    // Since this is in-memory, there's no need for fallback mechanisms
+    // to check multiple storage locations
   }
 }