fix: Convert integer timestamps to strings for database insertion

The APRS parser is sending timestamp as integer (Unix timestamp) but our
schema expects a string. This was causing Ecto.ChangeError in production
preventing packet insertion.

Convert integer timestamps to strings in the field conversion pipeline.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

lib/aprsme/packet_consumer.ex

@@ -450,7 +450,8 @@ defmodule Aprsme.PacketConsumer do
 
   # Convert field names that don't match our schema
   defp convert_field_names(attrs) do
-    then(attrs, fn a ->
+    attrs
+    |> then(fn a ->
       # Convert aprs_messaging? to aprs_messaging
       case Map.get(a, :aprs_messaging?) || Map.get(a, "aprs_messaging?") do
         nil ->
@@ -463,6 +464,16 @@ defmodule Aprsme.PacketConsumer do
           |> Map.delete("aprs_messaging?")
       end
     end)
+    |> then(fn a ->
+      # Convert timestamp from integer to string if needed
+      case Map.get(a, :timestamp) do
+        timestamp when is_integer(timestamp) ->
+          Map.put(a, :timestamp, Integer.to_string(timestamp))
+
+        _ ->
+          a
+      end
+    end)
   end
 
   # Convert latitude/longitude to lat/lon if present at top level