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/run.sh

@@ -0,0 +1,106 @@
+#!/usr/bin/env bash
+# WebDAV drive-root URL-scheme variant runner.
+#
+# Exercises `OXICLOUD_WEBDAV_DRIVE_LISTING_PREFIX=""` — the config where the
+# WebDAV `@drive` path segment is disabled and `/webdav/` IS the
+# drive listing. `tests/api/webdav_drive_root.hurl` covers the
+# default `"@drive"` config in the main API run; this runner
+# starts a separately-configured server to cover the empty-string
+# case, mirroring the OIDC runner's shape.
+#
+# Usage (from repo root):
+#   bash tests/webdav-drive-root/run.sh
+#
+# Prerequisites: docker, cargo, hurl ≥ 4.0
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+COMMON="$REPO_ROOT/tests/common"
+TEST_DIR="$REPO_ROOT/tests/webdav-drive-root"
+
+# shellcheck source=test.env
+source "$TEST_DIR/test.env"
+
+SERVER_PORT="${base_url##*:}"
+
+log()  { echo "[webdav-drive-root] $*"; }
+die()  { echo "[webdav-drive-root] ERROR: $*" >&2; exit 1; }
+
+wait_for_http() {
+  local url="$1" timeout="${2:-60}"
+  local deadline=$(( $(date +%s) + timeout ))
+  until curl -sf "$url" >/dev/null 2>&1; do
+    [[ $(date +%s) -ge $deadline ]] && die "Timeout waiting for $url"
+    sleep 1
+  done
+}
+
+# ── Teardown (always runs on exit) ────────────────────────────────────────────
+
+SERVER_PID=""
+
+cleanup() {
+  if [[ -n "$SERVER_PID" ]]; then
+    log "Stopping OxiCloud server (pid $SERVER_PID)..."
+    kill "$SERVER_PID" 2>/dev/null || true
+    wait "$SERVER_PID" 2>/dev/null || true
+  fi
+  bash "$COMMON/stop-db.sh"
+}
+
+trap cleanup EXIT
+
+# ── 1. Start postgres ─────────────────────────────────────────────────────────
+
+bash "$COMMON/spawn-db.sh"
+
+# ── 2. Load the drive-root-variant server env + port ──────────────────────────
+
+set -a
+# shellcheck source=../common/server-webdav-drive-root.env
+source "$COMMON/server-webdav-drive-root.env"
+OXICLOUD_SERVER_PORT=$SERVER_PORT
+OXICLOUD_STORAGE_PATH="$REPO_ROOT/tests/webdav-drive-root/storage"
+set +a
+
+# shellcheck source=../common/wipe-storage.sh
+source "$COMMON/wipe-storage.sh"
+wipe_storage "$OXICLOUD_STORAGE_PATH"
+
+# ── 3. Start OxiCloud server with the drive-root-variant config ───────────────
+
+BUILD_TARGET="${BUILD_TARGET:-debug}"
+OXICLOUD_BIN="$REPO_ROOT/target/$BUILD_TARGET/oxicloud"
+
+if [[ ! -x "$OXICLOUD_BIN" ]]; then
+  log "Building OxiCloud server ($BUILD_TARGET)..."
+  case "$BUILD_TARGET" in
+    debug)   (cd "$REPO_ROOT" && cargo build           2>&1 | tail -n 20) || die "cargo build failed" ;;
+    release) (cd "$REPO_ROOT" && cargo build --release 2>&1 | tail -n 20) || die "cargo build --release failed" ;;
+    *)       die "Unsupported BUILD_TARGET='$BUILD_TARGET' (expected 'debug' or 'release')" ;;
+  esac
+fi
+
+log "Starting OxiCloud server with WEBDAV_DRIVE_LISTING_PREFIX='' on port $SERVER_PORT..."
+"$OXICLOUD_BIN" --config "$COMMON/server-webdav-drive-root.env" &
+SERVER_PID=$!
+log "Waiting for server at $base_url..."
+wait_for_http "$base_url/ready" 120
+log "Server is ready."
+
+# ── 4. Run Hurl tests ─────────────────────────────────────────────────────────
+#
+# `setup.hurl` from the shared api/ suite bootstraps the initial admin
+# account via `POST /api/setup` — the endpoint locks after the first
+# admin exists, so it's a one-shot idempotency-by-server-state seed.
+# We reuse the file rather than duplicating the setup body so credential
+# / schema changes in the api tests automatically flow here.
+
+log "Running Hurl tests..."
+hurl --variables-file "$TEST_DIR/test.env" \
+     --file-root "$REPO_ROOT/tests" \
+     --test --jobs 1 \
+     "$REPO_ROOT/tests/api/setup.hurl" \
+     "$TEST_DIR/drive_root_empty_config.hurl"
+
+log "webdav-drive-root tests passed."