fix: drop doubled felhom-data segment from HDD app compose templates

Model A binds the guest mount /mnt/<drive> directly onto the host's
<drive>/felhom-data namespace, so the guest mount already IS felhom-data.
The templates' ${HDD_PATH}/felhom-data/appdata/<app> therefore double-nested
to <drive>/felhom-data/felhom-data/appdata/<app> on disk, diverging from the
provenance-aware backup helpers (NamespaceRoot(drive,true) -> single-nested).

Change all four HDD app templates (romm, nextcloud, immich, paperless-ngx)
to ${HDD_PATH}/appdata/<app>, matching AppDataDir(NamespaceRoot(HDD_PATH,true)).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

templates/nextcloud/docker-compose.yml

@@ -35,7 +35,7 @@ services:
       - REDIS_HOST=nextcloud-redis
     volumes:
       - nextcloud_html:/var/www/html
-      - ${HDD_PATH}/felhom-data/appdata/nextcloud:/var/www/html/data
+      - ${HDD_PATH}/appdata/nextcloud:/var/www/html/data
     networks:
       - traefik-public
       - nextcloud-internal