Fix symbol validation - prevent empty symbol codes

Added validation to ensure all packets with positions have valid symbols:
- symbol_code must be exactly 1 character, defaults to '>' (car) if missing
- symbol_table_id must be exactly 1 character, defaults to '/' if missing
- Validation only applies to packets with positions

Fixed 5 existing packets with empty symbol codes that would have caused
display issues on the map.

This prevents errors when rendering APRS symbols and ensures all positioned
packets can be displayed correctly.

lib/aprsme/packet.ex

@@ -147,6 +147,39 @@ defmodule Aprsme.Packet do
     changeset
     |> maybe_create_geometry_from_lat_lon()
     |> maybe_set_has_position()
+    |> normalize_symbols()
+  end
+
+  defp normalize_symbols(changeset) do
+    # Only normalize if packet has position
+    if get_field(changeset, :has_position) do
+      symbol_code = get_field(changeset, :symbol_code)
+      symbol_table_id = get_field(changeset, :symbol_table_id)
+
+      changeset
+      |> normalize_symbol_code(symbol_code)
+      |> normalize_symbol_table_id(symbol_table_id)
+    else
+      changeset
+    end
+  end
+
+  defp normalize_symbol_code(changeset, code) when is_binary(code) and byte_size(code) == 1 do
+    changeset
+  end
+
+  defp normalize_symbol_code(changeset, _invalid_code) do
+    # Default to '>' (car) if symbol_code is missing or invalid
+    put_change(changeset, :symbol_code, ">")
+  end
+
+  defp normalize_symbol_table_id(changeset, table) when is_binary(table) and byte_size(table) == 1 do
+    changeset
+  end
+
+  defp normalize_symbol_table_id(changeset, _invalid_table) do
+    # Default to '/' (primary symbol table) if table_id is missing or invalid
+    put_change(changeset, :symbol_table_id, "/")
   end
 
   @spec maybe_create_geometry_from_lat_lon(Ecto.Changeset.t()) :: Ecto.Changeset.t()