umami: read the admin password from its mounted secret file

The whole-mesh dry-run found umami's runtime crash-looping "admin password is
not set": its `admin` own-secret is mounted at /run/secrets/admin, but the
client read the bare env UMAMI_ADMIN_PASSWORD, which nothing sets. Same shape as
the six tool-runtime credential fixes — read the mounted file first
(MESH_UMAMI_ADMIN_PASSWORD_FILE), falling back to the env. (photos and mailu
remain deeper conversion jobs — a stub app image and a full Mailu config env —
not credential-wiring, tracked separately.)

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

modules/umami/module.json

@@ -110,7 +110,8 @@
       ],
       "env": {
         "MESH_BROKER_FILE": "/run/secrets/broker",
-        "MESH_RECEIVES": "/var/lib/umami/grants/mesh.json"
+        "MESH_RECEIVES": "/var/lib/umami/grants/mesh.json",
+        "MESH_UMAMI_ADMIN_PASSWORD_FILE": "/run/secrets/admin"
       },
       "env-file": [
         "/var/lib/umami/provisioner.env"