add User persistence: model, migration, eloquent + fake repo

UserModel maps users table (id, email unique, password_hash,
is_admin bool default false). EloquentUserRepository implements
UserRepository: create from CreateUserDto, find by id,
findByEmail. toDomain() materializes a User entity wrapping email
in EmailAddress vo. FakeUserRepository: in-memory map keyed by
auto-incrementing id, returns defensive copies on read (per
youngstartup pattern). composer stan script now passes
--no-progress for cleaner ci output.

backend/database/migrations/2026_05_06_000000_create_users_table.php

@@ -0,0 +1,23 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('users', function (Blueprint $table) {
+            $table->id();
+            $table->string('email')->unique();
+            $table->string('password_hash');
+            $table->boolean('is_admin')->default(false);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('users');
+    }
+};