Log data location and counts at startup; give the compose volume a fixed name

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -23,3 +23,4 @@ services:
       - hub-data:/data   # passkeys, calendar links and the session secret live here
 volumes:
   hub-data:
+    name: nehub-data   # fixed name so the data survives redeploys and renamed stacks