Give every API operation a unique operationId and surface scanner errors

Scanner status handlers all derived operationId "status" from their fn
name, so Swagger UI's "Try it out" executed the first one (ARP) — the
DHCP doc hit /api/arp_scanner/status. Same collision for read/list/
register/unregister. Each path now sets an explicit unique operation_id.

main's tokio::join!(...).0 kept only the ARP result and silently dropped
the other tasks' errors, so a DHCP scanner that failed to bind port 67
just showed "off" with no log. Each task is now wrapped to log its error.

Fixes #5

backend/src/web_server/config.rs

@@ -6,6 +6,7 @@ use crate::settings::get_settings;
 #[utoipa::path(
     get,
     path = "/api/config",
+    operation_id = "config_read",
     tag = "config",
     responses(
         (status = 200, description = "Front-end configuration", body = Config),