Consolidate backend scanner status, paging, and query-param duplication

Add ActiveStatusCell/PassiveStatusCell wrappers in the scanners common
module so the five per-scanner status.rs files reduce to a single static;
replace parse_parameter_bool/int/string with one generic parse_parameter
over FromStr; extract the shared LIMIT/OFFSET paging clause into
db::apply_paging; and drop a no-op for-loop in the ARP sender.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/web_server/dhcp_scanner.rs

@@ -13,5 +13,5 @@ use crate::web_server::scanner_status::{PassiveScannerStatusResponse, passive_re
     security(("bearer_auth" = []))
 )]
 pub async fn status() -> Result<Json<PassiveScannerStatusResponse>, StatusCode> {
-    passive_response(crate::scanners::dhcp::status::get()).map(Json)
+    passive_response(crate::scanners::dhcp::status::STATUS.get()).map(Json)
 }