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/lib/api/router.dart

@@ -0,0 +1,16 @@
+import 'dart:convert';
+
+import 'package:shelf/shelf.dart';
+import 'package:shelf_router/shelf_router.dart';
+
+Handler buildApiHandler() {
+  final router = Router()
+    ..get('/health', (Request request) {
+      return Response.ok(
+        jsonEncode({'status': 'ok'}),
+        headers: {'content-type': 'application/json'},
+      );
+    });
+
+  return const Pipeline().addMiddleware(logRequests()).addHandler(router.call);
+}