fix: run data-manager as the host operator's UID so hardlinks work both ways

`fs.protected_hardlinks=1` lets only a file's owner hardlink to it. The data-manager defaulted to UID 1001 (its image's `datamgr` user), the build helpers and the host CLI run as the operator (typically UID 1000), and any container the operator's UID launched (`--user 1000:1000` on the motis transitous-tools build) wrote to /data as 1000. Whoever wrote the file last owned it; whoever tried to link it next was almost always wrong. The motis build hit this on its config.yml — the data-manager API got HTTP 500 trying to hardlink an ubuntu-owned file from inside its own container. Pin the data-manager's runtime user to `${UID:-1000}:${GID:-1000}` so every actor writes as the same UID and `protected_hardlinks` becomes a no-op for the workflow. Override HOME to /tmp so tsx/python don't fight the image-baked /home/datamgr permissions.

Author: Medformatik

services/data-manager/service.json

@@ -9,7 +9,9 @@
     "image": "ghcr.io/medformatik/openmapx-data-manager",
     "tag": "latest",
     "expose": [4000],
+    "user": "${UID:-1000}:${GID:-1000}",
     "environment": {
+      "HOME": "/tmp",
       "DATA_DIR": "/data",
       "PORT": "4000",
       "HOST": "0.0.0.0",