Correct two false doc comments (closes #403)

Two doc comments described behaviour that no longer exists. The comment on `ExportSnapshotMetadata` described an SQL dump uploaded under `snapshots/`; it now lists the steps the function performs and the two keys it writes. The comment on `EnsureStorageBinding` named `vaultik database purge`; the command is `vaultik database delete`. Comments only.

Model: opus-5-5

internal/vaultik/storage_bind.go

@@ -30,7 +30,7 @@ var errStorageBindingMismatch = errors.New(
 //     nothing and return nil.
 //   - When the two differ, refuse with an error that tells the user
 //     how to recover (revert the config, or run `vaultik database
-//     purge` to discard the local index and rebuild against the new
+//     delete` to discard the local index and rebuild against the new
 //     destination on the next backup).
 //
 // Read-only inspection commands (remote info, snapshot list, etc.)