refactor: replace config.yml with .env for all configuration

All settings (auth credentials, scanner ranges, status_checker interval)
now live in a single .env file via pydantic-settings. config.yml and
config.yml.example are deleted.

- Settings: add auth_username, auth_password_hash, scanner_ranges,
  status_checker_interval; add load_overrides()/save_overrides() for
  persisting runtime changes to data/scan_config.json
- auth.py: read credentials directly from settings
- scan.py: read ranges/interval from settings; write-back via save_overrides()
- scheduler.py: read interval directly from settings
- main.py: call settings.load_overrides() at startup
- docker-compose.yml: remove config.yml volume mount, add new env vars
- conftest.py: set settings fields directly instead of writing a temp config.yml

backend/.env.example

@@ -1,4 +1,14 @@
 SECRET_KEY=<generate with: python3 -c "import secrets; print(secrets.token_urlsafe(32))">
 SQLITE_PATH=./data/homelab.db
-CONFIG_PATH=./config.yml
 CORS_ORIGINS=["http://localhost:5173","http://localhost:3000"]
+
+# Auth — set a strong password hash (generate with passlib):
+#   python3 -c "from passlib.context import CryptContext; print(CryptContext(schemes=['bcrypt']).hash('yourpassword'))"
+AUTH_USERNAME=admin
+AUTH_PASSWORD_HASH=
+
+# Scanner — JSON array of CIDR ranges to scan
+SCANNER_RANGES=["192.168.1.0/24"]
+
+# Status checker interval in seconds
+STATUS_CHECKER_INTERVAL=60