feat: Add comprehensive S3 storage testing with mocks

CRITICAL: Ensures production reliability at scale

- Add aws-sdk-client-mock for S3 testing without AWS resources
- Create comprehensive test suite for ALL S3 operations
- Test statistics persistence, recovery, and partitioning
- Test rate limiting, throttling, and retry logic
- Test concurrent operations from multiple services
- Test data integrity with checksums
- Test batch operations and performance optimizations
- Simulate millions of records for scale testing
- Ensure backward compatibility with legacy formats

Also: Remove deprecated addSmart() method (functionality merged into add())

These tests are critical for production deployments where S3 is the primary
storage backend for scaled systems handling millions of records.

package.json

@@ -196,6 +196,8 @@
     "@vitejs/plugin-basic-ssl": "^2.1.0",
     "@vitest/coverage-v8": "^3.2.4",
     "@vitest/ui": "^3.2.4",
+    "aws-sdk-client-mock": "^4.1.0",
+    "aws-sdk-client-mock-jest": "^4.1.0",
     "buffer": "^6.0.3",
     "crypto-browserify": "^3.12.1",
     "eslint": "^9.0.0",