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/scanners/mdns/status.rs

@@ -1,26 +1,3 @@
-use once_cell::sync::OnceCell;
-use std::sync::Mutex;
+use crate::scanners::common::passive_status::PassiveStatusCell;
 
-use crate::scanners::common::passive_status::{PassiveSnapshot, PassiveStatus};
-
-static STATUS: OnceCell<Mutex<PassiveStatus>> = OnceCell::new();
-
-pub fn init() {
-    STATUS.set(Mutex::new(PassiveStatus::new())).ok();
-}
-
-pub fn set_listening() {
-    if let Some(m) = STATUS.get() {
-        m.lock().unwrap().set_listening();
-    }
-}
-
-pub fn record_discovery(mac: &str) {
-    if let Some(m) = STATUS.get() {
-        m.lock().unwrap().record_discovery(mac);
-    }
-}
-
-pub fn get() -> Option<PassiveSnapshot> {
-    STATUS.get().map(|m| m.lock().unwrap().snapshot())
-}
+pub static STATUS: PassiveStatusCell = PassiveStatusCell::new();