types: add @type t to last two untyped GenServer structs

Every other defstruct in the app already has a @type t declaration;
these two GenServer states were the last holdouts. Declaring t here
means the whole codebase is ready for the typed-struct milestone of
Elixir's set-theoretic type system (the next phase after the inference
pass already shipping in 1.19).

lib/aprsme/connection_monitor.ex

@@ -22,6 +22,13 @@ defmodule Aprsme.ConnectionMonitor do
     :last_check
   ]
 
+  @type t :: %__MODULE__{
+          node_stats: %{optional(node()) => map()},
+          local_connections: non_neg_integer(),
+          draining: boolean(),
+          last_check: integer()
+        }
+
   def start_link(opts) do
     GenServer.start_link(__MODULE__, opts, name: __MODULE__)
   end

lib/aprsme_web/live/map_live/packet_batcher.ex

@@ -14,6 +14,12 @@ defmodule AprsmeWeb.MapLive.PacketBatcher do
 
   defstruct [:parent_pid, :buffer, :timer_ref]
 
+  @type t :: %__MODULE__{
+          parent_pid: pid(),
+          buffer: [map()],
+          timer_ref: reference() | nil
+        }
+
   @doc """
   Start the packet batcher for a LiveView process.
   """