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

example.env

@@ -85,6 +85,30 @@ OXICLOUD_SERVER_HOST=127.0.0.1
 # to an admin token. Default: false.
 #OXICLOUD_ENABLE_ADMIN_INTERNAL_ENDPOINTS=false
 
+# Native WebDAV URL segment that returns the drive listing. Sanitized
+# by trimming leading/trailing `/` so `/@drive/`, `@drive`, and
+# `@drive/` are equivalent. Three deployment modes:
+#
+#   * Default `@drive` — back-compat with pre-multi-drive clients.
+#       /webdav/…                  → caller's default personal drive
+#       /webdav/@drive/            → drive listing (per-drive virtual
+#                                    folders)
+#       /webdav/@drive/<sel>/…     → specific drive by UUID or its
+#                                    display name
+#
+#   * Empty `""` — no default-drive shortcut; `/webdav/` IS the
+#     drive listing. Clients must always name the drive.
+#       /webdav/                   → drive listing
+#       /webdav/<sel>/…            → specific drive
+#
+#   * Any other string (e.g. `drives`) — same shape as `@drive` but
+#     with your chosen segment substituted.
+#
+# Selector `<sel>` is a drive UUID or the drive's display name. Only
+# drives the caller has Read on via role_grants resolve; unknown
+# selector and permission denial both return 404 (anti-enumeration).
+#OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX=@drive
+
 # How often (milliseconds) the background job drains storage.tree_etag_dirty
 # and bumps folder tree ETags (default: 500). Write paths only enqueue bump
 # requests — this is the upper bound on how stale an ancestor folder's ETag