feat(server): schéma PostgreSQL sync-ready (ticket #49)

Ajoute la migration initiale (migrations/0001_initial_schema.sql,
incluant la table users nécessaire à l'adapter d'auth du ticket #48),
le runner de migration (bin/migrate.dart) et l'accès Postgres
(infrastructure/postgres/postgres_database.dart). dart pub get OK,
dart analyze clean, dart test vert (le test PostgreSQL réel est skip
faute de TEST_DATABASE_URL/Docker disponible dans ce sandbox ; SQL de
migration revu manuellement).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/test/postgres_migrations_test.dart

@@ -0,0 +1,48 @@
+import 'dart:io';
+
+import 'package:gametime_server/infrastructure/postgres/postgres_database.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('PostgreSQL migrations create the sync-ready schema', () async {
+    final databaseUrl = Platform.environment['TEST_DATABASE_URL'];
+    if (databaseUrl == null || databaseUrl.trim().isEmpty) {
+      markTestSkipped(
+        'Set TEST_DATABASE_URL to run PostgreSQL migration tests.',
+      );
+      return;
+    }
+
+    final connection = await PostgresConnectionFactory.fromDatabaseUrl(
+      databaseUrl,
+    ).open();
+    addTearDown(() => connection.close());
+
+    await PostgresMigrator(connection).applyMigrations();
+
+    final result = await connection.execute('''
+      SELECT table_name
+      FROM information_schema.tables
+      WHERE table_schema = 'public'
+        AND table_name IN (
+          'users',
+          'auth_sessions',
+          'synced_resources',
+          'shares',
+          'share_recipients'
+        )
+    ''');
+    final tableNames = result.map((row) => row[0] as String).toSet();
+
+    expect(
+      tableNames,
+      containsAll({
+        'users',
+        'auth_sessions',
+        'synced_resources',
+        'shares',
+        'share_recipients',
+      }),
+    );
+  });
+}