fix: handle unreadable admins.json in Docker and exclude from image

Use fs.accessSync to check read permissions before reading the config
file. If the file exists but isn't readable, log a warning and fall
through to the ADMIN_KEY fallback. Also add config/admins.json to
backend/.dockerignore to prevent it from being copied into the image.

Made-with: Cursor

backend/.dockerignore

@@ -0,0 +1,13 @@
+node_modules
+npm-debug.log
+.env
+.env.local
+*.db
+*.sqlite
+*.sqlite3
+data/
+.git
+.gitignore
+README.md
+config/admins.json
+