P15d: storage usage display + owner admin panel

- Server: AccountStore gains admin gating (ADMIN_USERS env), per-user quotaBytes
  override, listUsers + blobBytes. /api/usage now reports total bytes (backup blob +
  cloud characters) + an admin flag. Admin routes GET /api/admin/users and POST
  /api/admin/quota (gated). Quotas are tracked, not yet enforced. +1 test.
- Client: Settings cloud panel shows "Your cloud storage", and for an admin a users
  table with per-user usage + an editable quota (GB). compose sets ADMIN_USERS=nilsb.

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

deploy/ttrpg.compose.yml

@@ -15,6 +15,8 @@ services:
       - NODE_ENV=production
       - DATA_DIR=/data
       - ALLOWED_ORIGINS=https://ttrpg.briggen.dev
+      # comma-separated usernames that can see the admin panel (set to your account)
+      - ADMIN_USERS=nilsb
     volumes:
       - ttrpg-data:/data
     security_opt: