**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.

package.json

@@ -127,7 +127,9 @@
     "@types/uuid": "^10.0.0",
     "@typescript-eslint/eslint-plugin": "^7.4.0",
     "@typescript-eslint/parser": "^7.4.0",
+    "@vitest/coverage-v8": "^3.2.4",
     "@vitest/ui": "^3.2.4",
+    "dotenv": "^17.2.1",
     "eslint": "^8.57.0",
     "happy-dom": "^18.0.1",
     "jsdom": "^26.1.0",
@@ -136,8 +138,7 @@
     "rollup-plugin-terser": "^7.0.2",
     "tslib": "^2.6.2",
     "typescript": "^5.4.5",
-    "vitest": "^3.2.4",
-    "@vitest/coverage-v8": "^3.2.4"
+    "vitest": "^3.2.4"
   },
   "dependencies": {
     "@aws-sdk/client-s3": "^3.540.0",