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/snapshot.go

@@ -111,10 +111,11 @@ func newSnapshotListCommand() *cobra.Command {
 	var jsonOutput bool
 
 	cmd := &cobra.Command{
-		Use:   "list",
-		Short: "List all snapshots",
-		Long:  "Lists all snapshots with their ID, timestamp, and compressed size",
-		Args:  cobra.NoArgs,
+		Use:     "list",
+		Aliases: []string{"ls"},
+		Short:   "List all snapshots",
+		Long:    "Lists all snapshots with their ID, timestamp, and compressed size",
+		Args:    cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			// Use unified config resolution
 			configPath, err := ResolveConfigPath()
@@ -242,7 +243,16 @@ func newSnapshotVerifyCommand() *cobra.Command {
 		Use:   "verify <snapshot-id>",
 		Short: "Verify snapshot integrity",
 		Long:  "Verifies that all blobs referenced in a snapshot exist",
-		Args:  cobra.ExactArgs(1),
+		Args: func(cmd *cobra.Command, args []string) error {
+			if len(args) != 1 {
+				_ = cmd.Help()
+				if len(args) == 0 {
+					return fmt.Errorf("snapshot ID required")
+				}
+				return fmt.Errorf("expected 1 argument, got %d", len(args))
+			}
+			return nil
+		},
 		RunE: func(cmd *cobra.Command, args []string) error {
 			snapshotID := args[0]
 
@@ -266,7 +276,13 @@ func newSnapshotVerifyCommand() *cobra.Command {
 						lc.Append(fx.Hook{
 							OnStart: func(ctx context.Context) error {
 								go func() {
-									if err := v.VerifySnapshotWithOptions(snapshotID, opts); err != nil {
+									var err error
+									if opts.Deep {
+										err = v.RunDeepVerify(snapshotID, opts)
+									} else {
+										err = v.VerifySnapshotWithOptions(snapshotID, opts)
+									}
+									if err != nil {
 										if err != context.Canceled {
 											if !opts.JSON {
 												log.Error("Verification failed", "error", err)
@@ -302,17 +318,37 @@ func newSnapshotRemoveCommand() *cobra.Command {
 	opts := &vaultik.RemoveOptions{}
 
 	cmd := &cobra.Command{
-		Use:     "remove <snapshot-id>",
+		Use:     "remove [snapshot-id]",
 		Aliases: []string{"rm"},
-		Short:   "Remove a snapshot and its orphaned blobs",
-		Long: `Removes a snapshot and any blobs that are no longer referenced by other snapshots.
+		Short:   "Remove a snapshot from the local database",
+		Long: `Removes a snapshot from the local database.
 
-This command downloads manifests from all other snapshots to determine which blobs
-are still in use, then deletes any blobs that would become orphaned.`,
-		Args: cobra.ExactArgs(1),
+By default, only removes from the local database. Use --remote to also remove
+the snapshot metadata from remote storage.
+
+Note: This does NOT remove blobs. Use 'vaultik prune' to remove orphaned blobs
+after removing snapshots.
+
+Use --all --force to remove all snapshots.`,
+		Args: func(cmd *cobra.Command, args []string) error {
+			all, _ := cmd.Flags().GetBool("all")
+			if all {
+				if len(args) > 0 {
+					_ = cmd.Help()
+					return fmt.Errorf("--all cannot be used with a snapshot ID")
+				}
+				return nil
+			}
+			if len(args) != 1 {
+				_ = cmd.Help()
+				if len(args) == 0 {
+					return fmt.Errorf("snapshot ID required (or use --all --force)")
+				}
+				return fmt.Errorf("expected 1 argument, got %d", len(args))
+			}
+			return nil
+		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			snapshotID := args[0]
-
 			// Use unified config resolution
 			configPath, err := ResolveConfigPath()
 			if err != nil {
@@ -333,7 +369,13 @@ are still in use, then deletes any blobs that would become orphaned.`,
 						lc.Append(fx.Hook{
 							OnStart: func(ctx context.Context) error {
 								go func() {
-									if _, err := v.RemoveSnapshot(snapshotID, opts); err != nil {
+									var err error
+									if opts.All {
+										_, err = v.RemoveAllSnapshots(opts)
+									} else {
+										_, err = v.RemoveSnapshot(args[0], opts)
+									}
+									if err != nil {
 										if err != context.Canceled {
 											if !opts.JSON {
 												log.Error("Failed to remove snapshot", "error", err)
@@ -359,8 +401,10 @@ are still in use, then deletes any blobs that would become orphaned.`,
 	}
 
 	cmd.Flags().BoolVarP(&opts.Force, "force", "f", false, "Skip confirmation prompt")
-	cmd.Flags().BoolVar(&opts.DryRun, "dry-run", false, "Show what would be deleted without deleting")
-	cmd.Flags().BoolVar(&opts.JSON, "json", false, "Output deletion stats as JSON")
+	cmd.Flags().BoolVar(&opts.DryRun, "dry-run", false, "Show what would be removed without removing")
+	cmd.Flags().BoolVar(&opts.JSON, "json", false, "Output result as JSON")
+	cmd.Flags().BoolVar(&opts.Remote, "remote", false, "Also remove snapshot metadata from remote storage")
+	cmd.Flags().BoolVar(&opts.All, "all", false, "Remove all snapshots (requires --force)")
 
 	return cmd
 }