nextcloud: use a module-owned mesh-admin account instead of 'admin'

the migrated data has no literal 'admin' account — HAL's real admin login
is a personal account (jochens), not a generic one. Resetting that would
touch a real user's own credential, so the module gets its own dedicated
admin-group service account instead, same pattern as the minio per-module
service accounts. mesh-admin was created once by hand on novox to match
this manifest for the already-migrated data; a genuinely fresh install
seeds it automatically via NEXTCLOUD_ADMIN_USER/NEXTCLOUD_ADMIN_PASSWORD.

modules/nextcloud/module.json

@@ -65,7 +65,7 @@
       "type": "file",
       "path": "/var/lib/nextcloud-module/server.env",
       "mode": "0600",
-      "content": "POSTGRES_HOST=${bound:postgres-database:at}:${bound:postgres-database:port}\nPOSTGRES_DB=${bound:postgres-database:as}\nPOSTGRES_USER=${bound:postgres-database:as}\nPOSTGRES_PASSWORD=${secret:postgres-database}\nNEXTCLOUD_ADMIN_USER=admin\nNEXTCLOUD_ADMIN_PASSWORD=${secret:admin}\nOBJECTSTORE_S3_HOST=${bound:s3-bucket:at}\nOBJECTSTORE_S3_PORT=${bound:s3-bucket:port}\nOBJECTSTORE_S3_BUCKET=nextcloud\nOBJECTSTORE_S3_KEY=${bound:s3-bucket:as}\nOBJECTSTORE_S3_SECRET=${secret:s3-bucket}\nOBJECTSTORE_S3_SSL=false\nOBJECTSTORE_S3_USEPATH_STYLE=true\n"
+      "content": "POSTGRES_HOST=${bound:postgres-database:at}:${bound:postgres-database:port}\nPOSTGRES_DB=${bound:postgres-database:as}\nPOSTGRES_USER=${bound:postgres-database:as}\nPOSTGRES_PASSWORD=${secret:postgres-database}\nNEXTCLOUD_ADMIN_USER=mesh-admin\nNEXTCLOUD_ADMIN_PASSWORD=${secret:admin}\nOBJECTSTORE_S3_HOST=${bound:s3-bucket:at}\nOBJECTSTORE_S3_PORT=${bound:s3-bucket:port}\nOBJECTSTORE_S3_BUCKET=nextcloud\nOBJECTSTORE_S3_KEY=${bound:s3-bucket:as}\nOBJECTSTORE_S3_SECRET=${secret:s3-bucket}\nOBJECTSTORE_S3_SSL=false\nOBJECTSTORE_S3_USEPATH_STYLE=true\n"
     },
     {
       "id": "html",
@@ -113,6 +113,7 @@
         "MESH_BROKER_FILE": "/run/secrets/broker",
         "MESH_NEXTCLOUD_URL": "http://127.0.0.1:${port:80}",
         "MESH_NEXTCLOUD_CONFIG_FILE": "/run/config/config.json",
+        "MESH_NEXTCLOUD_ADMIN_USER": "mesh-admin",
         "MESH_NEXTCLOUD_ADMIN_PASSWORD_FILE": "/run/secrets/admin"
       },
       "restart-on": [