Ensure system actor user exists via migration

Creates user system@mila.local with Admin role if missing. Idempotent;
guarantees system actor in production without relying on seeds.

priv/repo/migrations/20260127124937_ensure_system_actor_user_exists.exs

@@ -0,0 +1,59 @@
+defmodule Mv.Repo.Migrations.EnsureSystemActorUserExists do
+  @moduledoc """
+  Ensures the system actor user always exists.
+
+  The system actor is used for systemic operations (email sync, cycle generation,
+  background jobs). It is created by seeds in development; in production seeds
+  may not run, so this migration guarantees the user exists.
+
+  Creates a user with email "system@mila.local" (default from Mv.Helpers.SystemActor)
+  and the Admin role. The user has no password and no OIDC ID, so it cannot log in.
+  """
+  use Ecto.Migration
+  import Ecto.Query
+
+  @system_user_email "system@mila.local"
+
+  def up do
+    admin_role_id = ensure_admin_role_exists()
+    ensure_system_actor_user_exists(admin_role_id)
+  end
+
+  def down do
+    # Not reversible - do not delete system user on rollback
+    :ok
+  end
+
+  defp ensure_admin_role_exists do
+    case repo().one(from(r in "roles", where: r.name == "Admin", select: r.id)) do
+      nil ->
+        execute("""
+        INSERT INTO roles (id, name, description, permission_set_name, is_system_role, inserted_at, updated_at)
+        VALUES (uuid_generate_v7(), 'Admin', 'Administrator with full access', 'admin', false, (now() AT TIME ZONE 'utc'), (now() AT TIME ZONE 'utc'))
+        """)
+
+        role_id = repo().one(from(r in "roles", where: r.name == "Admin", select: r.id))
+        IO.puts("✅ Created 'Admin' role (was missing)")
+        role_id
+
+      role_id ->
+        role_id
+    end
+  end
+
+  defp ensure_system_actor_user_exists(_admin_role_id) do
+    case repo().one(from(u in "users", where: u.email == ^@system_user_email, select: u.id)) do
+      nil ->
+        execute("""
+        INSERT INTO users (id, email, hashed_password, oidc_id, member_id, role_id)
+        SELECT gen_random_uuid(), '#{@system_user_email}', NULL, NULL, NULL, r.id
+        FROM roles r WHERE r.name = 'Admin' LIMIT 1
+        """)
+
+        IO.puts("✅ Created system actor user (#{@system_user_email})")
+
+      _ ->
+        :ok
+    end
+  end
+end