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/README.md

@@ -24,6 +24,39 @@ When `PORT` is not set, it listens on `8080`.
 PORT=9090 dart run bin/server.dart
 ```
 
+## PostgreSQL
+
+Ticket #49 adds the initial sync-ready PostgreSQL schema and a minimal
+connection/migration utility.
+
+Database environment variables:
+
+- `DATABASE_HOST`: PostgreSQL host.
+- `DATABASE_PORT`: PostgreSQL port, defaults to `5432` when omitted.
+- `DATABASE_NAME`: database name.
+- `DATABASE_USER`: database user.
+- `DATABASE_PASSWORD`: database password.
+
+Apply migrations from `server/`:
+
+```bash
+DATABASE_HOST=localhost \
+DATABASE_PORT=5432 \
+DATABASE_NAME=gametime \
+DATABASE_USER=gametime \
+DATABASE_PASSWORD=gametime \
+dart run bin/migrate.dart
+```
+
+Migrations are read from `server/migrations/` in alphabetical order. The v1
+runner is intentionally simple; SQL files use idempotent DDL where practical.
+
+The PostgreSQL integration test is skipped unless `TEST_DATABASE_URL` is set:
+
+```bash
+TEST_DATABASE_URL=postgres://gametime:gametime@localhost:5432/gametime dart test
+```
+
 Healthcheck:
 
 ```bash
@@ -39,7 +72,8 @@ Expected response:
 ## Scope
 
 Ticket #47 only scaffolds the Dart server, the hexagonal directory layout and
-the `/health` endpoint.
+the `/health` endpoint. Ticket #49 adds the first PostgreSQL schema only; auth,
+sync endpoints and sharing behavior are still implemented in later tickets.
 
 Upcoming tickets will fill the empty adapters and use cases: