fix: consolidate .env.example to root, add missing auth defaults

- Add AUTH_USERNAME, AUTH_PASSWORD_HASH (admin/admin) to root .env.example
- Remove duplicate backend/.env.example — root file is the single source of truth
- Fix fresh-install login failure caused by missing password hash

.env.example

@@ -1,5 +1,16 @@
 # Backend - server-side only (NEVER commit .env)
 SECRET_KEY=change_me_in_production
 SQLITE_PATH=./data/homelab.db
-CONFIG_PATH=./config.yml
-CORS_ORIGINS=http://localhost:5173
+CORS_ORIGINS=["http://localhost:5173","http://localhost:3000"]
+
+# Auth — default credentials: admin / admin
+# ⚠️  Change before exposing on a network.
+# Generate a new hash: python3 -c "from passlib.context import CryptContext; print(CryptContext(schemes=['bcrypt']).hash('yourpassword'))"
+AUTH_USERNAME=admin
+AUTH_PASSWORD_HASH=$2b$12$RtMbyw17l4N5UGzeXMNAWuzCaVV.XFBY7ZetWheQhxcBDcxahapkG
+
+# Scanner — JSON array of CIDR ranges to scan
+SCANNER_RANGES=["192.168.1.0/24"]
+
+# Status checker interval in seconds
+STATUS_CHECKER_INTERVAL=60