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/README.md

@@ -0,0 +1,51 @@
+# GameTime server
+
+Headless Dart server for future GameTime account, sync and sharing features.
+This package is separate from the Flutter app at the repository root.
+
+## Local run
+
+Install dependencies from this directory:
+
+```bash
+dart pub get
+```
+
+Run the server:
+
+```bash
+dart run bin/server.dart
+```
+
+The HTTP server binds to `0.0.0.0` and reads `PORT` from the environment.
+When `PORT` is not set, it listens on `8080`.
+
+```bash
+PORT=9090 dart run bin/server.dart
+```
+
+Healthcheck:
+
+```bash
+curl http://localhost:8080/health
+```
+
+Expected response:
+
+```json
+{"status":"ok"}
+```
+
+## Scope
+
+Ticket #47 only scaffolds the Dart server, the hexagonal directory layout and
+the `/health` endpoint.
+
+Upcoming tickets will fill the empty adapters and use cases:
+
+- #48: user authentication and API tokens.
+- #49: PostgreSQL schema and repositories.
+- #50: incremental sync API.
+- #51: targeted sharing.
+- #52: Docker and registry packaging.
+- #53: API, contract and integration tests.