Add blocklist candidate listing, scan trigger, status update,
push/unblock to Pi-hole, and connection test endpoints.
Add pihole_url/version/api_key and router_source_ids/device_names
fields to SettingsBody and prefs handling in patch_settings.
Add PiholeClient.__post_init__ validation so 503 fires naturally
when url/api_key are unconfigured (mock-safe: bypassed in tests).
