deploy-remote: migrate_data_root merges into pre-seeded destinations

mv of a data root onto a non-empty directory fails (Directory not empty).
The romm/copyparty cutover creates Containers/<svc>/ before migrating:
the seed env file lands there first, and the romm anonymous-volume rescue
extracts into it before the /opt move. Merge the old root's entries in
without overwriting existing state (mv -n), warn and leave residue if a
collision kept the new side, and keep the plain rename when the
destination is absent or empty.

scripts/deploy-remote.sh

@@ -534,10 +534,24 @@ wait_container_healthy() {
 # the unit; re-running the handler on an already-active unit restarts only the
 # selected unit.
 
-migrate_data_root() { # $1 = old /opt path, $2 = new data-root path (same fs: rename)
-    local from="$1" to="$2"
+migrate_data_root() { # $1 = old /opt path, $2 = new data-root path (same fs: rename/merge)
+    local from="$1" to="$2" entry
     [[ -d "$from" ]] || return 0
-    mv -- "$from" "$to"
+    if [[ -d "$to" && -n "$(ls -A "$to" 2>/dev/null)" ]]; then
+        # The destination already holds seeded state (env file, rescued
+        # anonymous-volume data). Merge the old root into it without ever
+        # overwriting an existing entry, then drop the emptied source.
+        shopt -s dotglob
+        for entry in "$from"/*; do
+            [[ -e "$entry" ]] || continue
+            mv -n -- "$entry" "$to/"
+        done
+        shopt -u dotglob
+        rmdir -- "$from" 2>/dev/null || \
+            echo "WARN: $from not empty after merge (collisions kept old side); left in place" >&2
+    else
+        mv -- "$from" "$to"
+    fi
     chown -R "$(id -u):$(id -g)" "$to"
 }