Test Drive Write

Prove write access to the configured filing root before enabling live filing: upload a uniquely-named marker file into drive_root_folder_id, then trash it (recoverable ~30 days; never a hard delete).

The read-only browse endpoint proves impersonation + read; on a shared drive only an actual write proves the impersonated user's membership role allows file creation. Every attempt is recorded in drive_write_test_log.