feat(drive): fix webdav back-compat

add env variable `OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX`
    which is by default:
    `OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX="@drive"`

    so `/webdav/` -> points to user's personal drive (**backward compatibilit**y)
    `/web/dav/@drive/{uuid|drive name}/` points to the respective drive

    if admins want directly `/webdav/` pointing to list of drives they need to:
    `OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX=""`

    + ensure lock is per user (RFC 4918 §9.11)

    fix: #554

tests/webdav-drive-root/test.env

@@ -0,0 +1,9 @@
+# Test credentials for the WebDAV drive-root variant runner — NOT real secrets.
+# Runs on a separate port from tests/api and tests/webdav so a
+# `just api-test` chain doesn't collide when the previous runner's
+# teardown is still in progress.
+base_url=http://localhost:8089
+username=admin
+email=admin@example.com
+# gitguardian:ignore
+password=TestPassword1!