fix: decode beacons_json before mutating in patch/remove helpers

patch_beacons_json/2 and remove_from_beacons_json/2 were calling
Enum.reject/2 directly on @beacons_json, which is a JSON-encoded
string from Jason.encode!/0 (not a list). This crashed with
Protocol.UndefinedError when no approved beacons existed (string
was "[]") and a PubSub broadcast triggered an update.

Decode with Jason.decode!/1, operate on the list, then re-encode
with Jason.encode!/1 so the template's data-beacons attribute
remains a valid JSON string for the JS hook.

lib/microwaveprop_web/live/beacon_live/index.ex

@@ -139,14 +139,16 @@ defmodule MicrowavepropWeb.BeaconLive.Index do
   # Replace or add a beacon in the in-memory JSON list without re-querying the DB.
   defp patch_beacons_json(%{assigns: %{beacons_json: json}} = socket, beacon) do
     entry = encode_single_beacon(beacon)
-    updated = Enum.reject(json, &(Map.get(&1, "id") == Map.get(entry, "id"))) ++ [entry]
-    assign(socket, :beacons_json, updated)
+    list = Jason.decode!(json)
+    updated = Enum.reject(list, &(Map.get(&1, "id") == Map.get(entry, "id"))) ++ [entry]
+    assign(socket, :beacons_json, Jason.encode!(updated))
   end
 
   defp remove_from_beacons_json(%{assigns: %{beacons_json: json}} = socket, beacon) do
     id_str = beacon.id |> Ecto.UUID.cast!() |> to_string()
-    updated = Enum.reject(json, &(Map.get(&1, "id") == id_str))
-    assign(socket, :beacons_json, updated)
+    list = Jason.decode!(json)
+    updated = Enum.reject(list, &(Map.get(&1, "id") == id_str))
+    assign(socket, :beacons_json, Jason.encode!(updated))
   end
 
   defp encode_single_beacon(beacon) do