feat: add S3 storage, locking, concatenation, and JSON metadata serialization

[tomdesair]

Summary

This pull request introduces full S3-compatible object storage support (S3StorageService), distributed locking (S3LockingService), dual-strategy concatenation (S3ConcatenationService), optional JSON metadata serialization, and developer instructions for Testcontainers + MinIO (supporting Docker and Podman).

Key Highlights

• S3StorageService: Always-multipart upload strategy matching tusd, incomplete part buffering (.part objects for sub-5MB chunks), dynamic part scaling up to 5GB, checksum deduplication, and zero-byte upload support.
• S3LockingService: S3 conditional writes (If-None-Match: "*"), heartbeat daemon lease renewals, cross-pod lock contention resolution (.stop signal files), enabling multi-replica container deployments without external Redis/database lockers.
• S3ConcatenationService: Fast server-side copy (UploadPartCopy) for partial uploads >= 5 MB and streaming re-upload fallback for sub-5MB chunks.
• JSON Serialization: Added .withJsonSerialization() builder option for TusFileUploadService and DiskStorageService.
• Integration Tests & Developer Docs: Added ITS3StorageService, ITS3LockingService, ITS3TusFileUploadService, and ITS3RufhProtocol integration test suites running against MinIO using TestUtils (supporting Docker & Podman). Added developer documentation in docs/S3_STORAGE.md.

Verification

• All 47 integration test scenarios pass on MinIO (0 skipped).
• 100% JaCoCo code coverage on new and modified lines.

[coveralls]

coverage: 95.854% (-0.6%) from 96.487% — feature/s3-storage-support into master