Add deterministic deduplication, rclone backend, and database purge command

- Implement deterministic blob hashing using double SHA256 of uncompressed
  plaintext data, enabling deduplication even after local DB is cleared
- Add Stat() check before blob upload to skip existing blobs in storage
- Add rclone storage backend for additional remote storage options
- Add 'vaultik database purge' command to erase local state DB
- Add 'vaultik remote check' command to verify remote connectivity
- Show configured snapshots in 'vaultik snapshot list' output
- Skip macOS resource fork files (._*) when listing remote snapshots
- Use multi-threaded zstd compression (CPUs - 2 threads)
- Add writer tests for double hashing behavior

internal/s3/client.go

@@ -10,6 +10,7 @@ import (
 	"github.com/aws/aws-sdk-go-v2/credentials"
 	"github.com/aws/aws-sdk-go-v2/feature/s3/manager"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
+	"github.com/aws/smithy-go/logging"
 )
 
 // Client wraps the AWS S3 client for vaultik operations.
@@ -35,12 +36,18 @@ type Config struct {
 	Region          string
 }
 
+// nopLogger is a logger that discards all output.
+// Used to suppress SDK warnings about checksums.
+type nopLogger struct{}
+
+func (nopLogger) Logf(classification logging.Classification, format string, v ...interface{}) {}
+
 // NewClient creates a new S3 client with the provided configuration.
 // It establishes a connection to the S3-compatible storage service and
 // validates the credentials. The client uses static credentials and
 // path-style URLs for compatibility with various S3-compatible services.
 func NewClient(ctx context.Context, cfg Config) (*Client, error) {
-	// Create AWS config
+	// Create AWS config with a nop logger to suppress SDK warnings
 	awsCfg, err := config.LoadDefaultConfig(ctx,
 		config.WithRegion(cfg.Region),
 		config.WithCredentialsProvider(credentials.NewStaticCredentialsProvider(
@@ -48,6 +55,7 @@ func NewClient(ctx context.Context, cfg Config) (*Client, error) {
 			cfg.SecretAccessKey,
 			"",
 		)),
+		config.WithLogger(nopLogger{}),
 	)
 	if err != nil {
 		return nil, err