Add passive mDNS/Bonjour discovery scanner

Introduce a second discovery module that passively listens for mDNS
multicast announcements (224.0.0.251:5353) and feeds discovered devices
into the existing devices/events/notifications pipeline, running in its
own task alongside the ARP scanner.

Since mDNS carries an IP and hostname but no MAC (the device key), the
module resolves IP->MAC via the OS ARP cache with a targeted ARP-probe
fallback. The advertised hostname is stored in a new optional `name`
column on devices (blank for ARP-only devices); the single `update`
writes `name` only when set so ARP rescans never clobber it. Device
names are included in event/notification messages.

Adds a GET /api/mdns_scanner/status endpoint (is_listening, devices_seen,
last_device_seen_seconds_ago) wired into OpenAPI, an optional
[mdns_scanner] config section, and the corresponding Nix module option.
Also aligns the Nix module's stale `timings` section with the current
`arp_scanner` schema.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/Cargo.toml

@@ -28,3 +28,5 @@ once_cell = "1.21.3"
 tower = "0.5.3"
 utoipa = { version = "5", features = ["axum_extras", "chrono"] }
 utoipa-swagger-ui = { version = "9", features = ["axum"] }
+simple-dns = "0.11.3"
+socket2 = "0.6.4"