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/cli/store.go

@@ -23,7 +23,7 @@ func NewStoreCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "store",
 		Short: "Storage information commands",
-		Long:  "Commands for viewing information about the S3 storage backend",
+		Long:  "Commands for viewing information about the storage backend",
 	}
 
 	// Add subcommands
@@ -37,7 +37,7 @@ func newStoreInfoCommand() *cobra.Command {
 	return &cobra.Command{
 		Use:   "info",
 		Short: "Display storage information",
-		Long:  "Shows S3 bucket configuration and storage statistics including snapshots and blobs",
+		Long:  "Shows storage configuration and statistics including snapshots and blobs",
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runWithApp(cmd.Context(), func(app *StoreApp) error {
 				return app.Info(cmd.Context())