feat: add referred_by_id to talents table

- Introduced a new migration to add the referred_by_id field to the talents table, establishing a reference to the users table with a nilify_all delete strategy.
- Created an index on the referred_by_id field for improved query performance.

Author: zcesur

priv/repo/migrations/20250708064941_add_referred_by_to_talents.exs

@@ -0,0 +1,11 @@
+defmodule Algora.Repo.Migrations.AddReferredByToTalents do
+  use Ecto.Migration
+
+  def change do
+    alter table(:talents) do
+      add :referred_by_id, references(:users, on_delete: :nilify_all)
+    end
+
+    create index(:talents, [:referred_by_id])
+  end
+end