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

docker-compose.yml

@@ -7,11 +7,13 @@ services:
     environment:
       SECRET_KEY: ${SECRET_KEY:?SECRET_KEY must be set in the environment or a .env file}
       SQLITE_PATH: /app/data/homelab.db
-      CONFIG_PATH: /app/config.yml
       CORS_ORIGINS: '["http://localhost:3000"]'
+      AUTH_USERNAME: ${AUTH_USERNAME:-admin}
+      AUTH_PASSWORD_HASH: ${AUTH_PASSWORD_HASH:?AUTH_PASSWORD_HASH must be set}
+      SCANNER_RANGES: ${SCANNER_RANGES:-["192.168.1.0/24"]}
+      STATUS_CHECKER_INTERVAL: ${STATUS_CHECKER_INTERVAL:-60}
     volumes:
       - backend_data:/app/data
-      - ./backend/config.yml:/app/config.yml
     networks:
       - homelable
     # Required for ping-based status checks