Fix callsign registration to be case-insensitive

Users could register duplicate callsigns with different casing (e.g.,
"w1aw" and "W1AW") because the database unique index was case-sensitive.
Additionally, live validation showed false "already taken" errors on
every keystroke.

Changes:
- Add case-insensitive unique index using lower(callsign)
- Skip callsign uniqueness validation during live form changes
- Add test verifying case-insensitive uniqueness enforcement

lib/aprsme/accounts.ex

@@ -103,7 +103,7 @@ defmodule Aprsme.Accounts do
 
   """
   def change_user_registration(%User{} = user, attrs \\ %{}) do
-    User.registration_changeset(user, attrs, hash_password: false, validate_email: false)
+    User.registration_changeset(user, attrs, hash_password: false, validate_email: false, validate_callsign: false)
   end
 
   ## Settings

priv/repo/migrations/20260219191058_make_callsign_case_insensitive.exs

@@ -0,0 +1,11 @@
+defmodule Aprsme.Repo.Migrations.MakeCallsignCaseInsensitive do
+  use Ecto.Migration
+
+  def change do
+    # Drop the existing case-sensitive unique index
+    drop unique_index(:users, [:callsign])
+
+    # Create a case-insensitive unique index using LOWER()
+    create unique_index(:users, ["lower(callsign)"], name: :users_callsign_lower_index)
+  end
+end

test/aprsme_web/live/user_registration_live_test.exs

@@ -69,6 +69,26 @@ defmodule AprsmeWeb.UserRegistrationLiveTest do
 
       assert result =~ "has already been taken"
     end
+
+    test "renders errors for duplicated callsign (case-insensitive)", %{conn: conn} do
+      {:ok, lv, _html} = live(conn, ~p"/users/register", on_error: :warn)
+
+      _user = user_fixture(%{callsign: "W1AW"})
+
+      # Try to register with same callsign in different case
+      result =
+        lv
+        |> form("#registration_form",
+          user: %{
+            "email" => unique_user_email(),
+            "callsign" => "w1aw",
+            "password" => "valid_password123"
+          }
+        )
+        |> render_submit()
+
+      assert result =~ "has already been taken"
+    end
   end
 
   describe "registration navigation" do