feat(server): scaffolding du serveur Dart headless hexagonal (ticket #47)

Ajoute le nouveau paquet server/ (architecture hexagonale : api,
domain, application, infrastructure/postgres, infrastructure/security,
migrations, scripts) avec un premier endpoint GET /health. dart pub
get OK, dart analyze clean, dart test 1/1 vert, serveur démarré
manuellement et validé (GET /health -> 200 {"status":"ok"}).

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

server/test/health_test.dart

@@ -0,0 +1,18 @@
+import 'dart:convert';
+
+import 'package:gametime_server/api/router.dart';
+import 'package:shelf/shelf.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('GET /health returns ok status', () async {
+    final handler = buildApiHandler();
+    final response = await handler(
+      Request('GET', Uri.parse('http://localhost/health')),
+    );
+
+    expect(response.statusCode, 200);
+    expect(response.headers['content-type'], contains('application/json'));
+    expect(jsonDecode(await response.readAsString()), {'status': 'ok'});
+  });
+}