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/pubspec.lock

@@ -41,6 +41,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.2"
+  buffer:
+    dependency: transitive
+    description:
+      name: buffer
+      sha256: "389da2ec2c16283c8787e0adaede82b1842102f8c8aae2f49003a766c5c6b3d1"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.2.3"
+  charcode:
+    dependency: transitive
+    description:
+      name: charcode
+      sha256: fb0f1107cac15a5ea6ef0a6ef71a807b9e4267c713bb93e00e92d737cc8dbd8a
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.4.0"
   cli_config:
     dependency: transitive
     description:
@@ -209,6 +225,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.5.2"
+  postgres:
+    dependency: "direct main"
+    description:
+      name: postgres
+      sha256: "123de5cbadc56a7e8d9fa485c780b6b56940b4081f4c74f3a5578682757c299b"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.5.12"
   pub_semver:
     dependency: transitive
     description: