fix: remove destructive sync from ListSnapshots

ListSnapshots() silently deleted local snapshot records not found in
remote storage. A list/read operation should not have destructive side
effects.

Changes:
- Remove the inline deletion loop from ListSnapshots() that deleted
  local snapshots not present in remote storage
- Update syncWithRemote() to use deleteSnapshotFromLocalDB() for
  proper cascade cleanup (deleting snapshot_files, snapshot_blobs,
  and snapshot_uploads before the snapshot record itself)

The sync behavior remains available via syncWithRemote(), which is
called explicitly by PurgeSnapshots().

closes #15

internal/vaultik/snapshot.go

@@ -361,33 +361,6 @@ func (v *Vaultik) ListSnapshots(jsonOutput bool) error {
 		localSnapshotMap[s.ID.String()] = s
 	}
 
-	// Remove local snapshots that don't exist remotely
-	for _, snapshot := range localSnapshots {
-		snapshotIDStr := snapshot.ID.String()
-		if !remoteSnapshots[snapshotIDStr] {
-			log.Info("Removing local snapshot not found in remote", "snapshot_id", snapshot.ID)
-
-			// Delete related records first to avoid foreign key constraints
-			if err := v.Repositories.Snapshots.DeleteSnapshotFiles(v.ctx, snapshotIDStr); err != nil {
-				log.Error("Failed to delete snapshot files", "snapshot_id", snapshot.ID, "error", err)
-			}
-			if err := v.Repositories.Snapshots.DeleteSnapshotBlobs(v.ctx, snapshotIDStr); err != nil {
-				log.Error("Failed to delete snapshot blobs", "snapshot_id", snapshot.ID, "error", err)
-			}
-			if err := v.Repositories.Snapshots.DeleteSnapshotUploads(v.ctx, snapshotIDStr); err != nil {
-				log.Error("Failed to delete snapshot uploads", "snapshot_id", snapshot.ID, "error", err)
-			}
-
-			// Now delete the snapshot itself
-			if err := v.Repositories.Snapshots.Delete(v.ctx, snapshotIDStr); err != nil {
-				log.Error("Failed to delete local snapshot", "snapshot_id", snapshot.ID, "error", err)
-			} else {
-				log.Info("Deleted local snapshot not found in remote", "snapshot_id", snapshot.ID)
-				delete(localSnapshotMap, snapshotIDStr)
-			}
-		}
-	}
-
 	// Build final snapshot list
 	snapshots := make([]SnapshotInfo, 0, len(remoteSnapshots))
 
@@ -802,7 +775,7 @@ func (v *Vaultik) syncWithRemote() error {
 		snapshotIDStr := snapshot.ID.String()
 		if !remoteSnapshots[snapshotIDStr] {
 			log.Info("Removing local snapshot not found in remote", "snapshot_id", snapshot.ID)
-			if err := v.Repositories.Snapshots.Delete(v.ctx, snapshotIDStr); err != nil {
+			if err := v.deleteSnapshotFromLocalDB(snapshotIDStr); err != nil {
 				log.Error("Failed to delete local snapshot", "snapshot_id", snapshot.ID, "error", err)
 			} else {
 				removedCount++