fix: recover durable route paths after restart

docs/deployment-preflight.md

@@ -89,6 +89,10 @@ python3 scripts/preflight-deployment.py ... \
 - A real `link(2)` operation between a unique zero-byte file in the qB root
   and one in the future automatic-route root. The probe verifies that both
   names refer to the same inode and removes them unconditionally.
+- Every existing `archive-control:*` Syncthing folder beneath `routes/` still
+  has its local directory. This catches a route-root bind-mount migration that
+  would otherwise hide an already configured folder and later fail a job with
+  `No such file or directory`.
 - The token, qB password, and Syncthing API-key files are non-empty regular
   files with no group/world permissions.
 - The client image can read its configuration and reports usable permissions,
@@ -102,6 +106,12 @@ root and add a `local_path_overrides` mapping for that exact API path. This
 prevents automatic route folders being created on a small configuration
 filesystem while the client expects to hardlink from the qB data mount.
 
+When converting an existing node, stop its client and Syncthing containers,
+move each existing `routes/<route-id>` directory from the old Syncthing config
+tree into the new qB-backed route-root directory, then recreate Syncthing and
+run this preflight. Do not discard existing route directories: they can contain
+route handshake state or an in-progress transfer namespace.
+
 ## Failure handling
 
 Treat a nonzero exit status as a deployment blocker. Correct the compose