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/lib/infrastructure/postgres/README.md

@@ -1,5 +1,6 @@
 # PostgreSQL infrastructure
 
-Placeholder for PostgreSQL repository adapters.
+Connection and migration utilities for server-side PostgreSQL.
 
-The schema and concrete persistence code are planned for ticket #49.
+Repository adapters will be added by the sync/auth tickets once their ports are
+defined.

server/lib/infrastructure/postgres/postgres_database.dart

@@ -0,0 +1,176 @@
+import 'dart:io';
+
+import 'package:postgres/postgres.dart';
+
+final class PostgresConnectionConfig {
+  const PostgresConnectionConfig({
+    required this.host,
+    required this.port,
+    required this.database,
+    required this.username,
+    required this.password,
+    this.sslMode = SslMode.disable,
+  });
+
+  factory PostgresConnectionConfig.fromEnvironment([
+    Map<String, String>? environment,
+  ]) {
+    final env = environment ?? Platform.environment;
+    return PostgresConnectionConfig(
+      host: _requiredEnv(env, 'DATABASE_HOST'),
+      port: int.tryParse(env['DATABASE_PORT'] ?? '') ?? 5432,
+      database: _requiredEnv(env, 'DATABASE_NAME'),
+      username: _requiredEnv(env, 'DATABASE_USER'),
+      password: _requiredEnv(env, 'DATABASE_PASSWORD'),
+    );
+  }
+
+  final String host;
+  final int port;
+  final String database;
+  final String username;
+  final String password;
+  final SslMode sslMode;
+}
+
+final class PostgresConnectionFactory {
+  const PostgresConnectionFactory(this.config) : databaseUrl = null;
+
+  const PostgresConnectionFactory.fromDatabaseUrl(this.databaseUrl)
+    : config = null;
+
+  final PostgresConnectionConfig? config;
+  final String? databaseUrl;
+
+  Future<Connection> open() {
+    final databaseUrl = this.databaseUrl;
+    if (databaseUrl != null) {
+      return Connection.openFromUrl(databaseUrl);
+    }
+    final config = this.config!;
+    return Connection.open(
+      Endpoint(
+        host: config.host,
+        port: config.port,
+        database: config.database,
+        username: config.username,
+        password: config.password,
+      ),
+      settings: ConnectionSettings(sslMode: config.sslMode),
+    );
+  }
+}
+
+final class PostgresMigrator {
+  const PostgresMigrator(this.connection);
+
+  final Connection connection;
+
+  Future<void> applyMigrations({String migrationsPath = 'migrations'}) async {
+    final directory = Directory(migrationsPath);
+    if (!await directory.exists()) {
+      throw StateError('Migrations directory not found: $migrationsPath');
+    }
+
+    final files =
+        await directory
+              .list()
+              .where((entity) => entity is File && entity.path.endsWith('.sql'))
+              .cast<File>()
+              .toList()
+          ..sort((left, right) => left.path.compareTo(right.path));
+
+    for (final file in files) {
+      final sql = await file.readAsString();
+      for (final statement in _splitSqlStatements(sql)) {
+        await connection.execute(statement);
+      }
+    }
+  }
+}
+
+String _requiredEnv(Map<String, String> environment, String key) {
+  final value = environment[key];
+  if (value == null || value.trim().isEmpty) {
+    throw StateError('Missing required environment variable: $key');
+  }
+  return value;
+}
+
+List<String> _splitSqlStatements(String sql) {
+  final statements = <String>[];
+  final buffer = StringBuffer();
+  var inSingleQuote = false;
+  var inDoubleQuote = false;
+  String? dollarQuoteTag;
+
+  for (var index = 0; index < sql.length; index++) {
+    final char = sql[index];
+    final next = index + 1 < sql.length ? sql[index + 1] : null;
+
+    if (dollarQuoteTag != null) {
+      if (sql.startsWith(dollarQuoteTag, index)) {
+        buffer.write(dollarQuoteTag);
+        index += dollarQuoteTag.length - 1;
+        dollarQuoteTag = null;
+      } else {
+        buffer.write(char);
+      }
+      continue;
+    }
+
+    if (!inSingleQuote && !inDoubleQuote && char == r'$') {
+      final tag = _readDollarQuoteTag(sql, index);
+      if (tag != null) {
+        dollarQuoteTag = tag;
+        buffer.write(tag);
+        index += tag.length - 1;
+        continue;
+      }
+    }
+
+    if (!inDoubleQuote && char == "'") {
+      buffer.write(char);
+      if (inSingleQuote && next == "'") {
+        index++;
+        buffer.write(next);
+      } else {
+        inSingleQuote = !inSingleQuote;
+      }
+      continue;
+    }
+
+    if (!inSingleQuote && char == '"') {
+      inDoubleQuote = !inDoubleQuote;
+      buffer.write(char);
+      continue;
+    }
+
+    if (!inSingleQuote && !inDoubleQuote && char == ';') {
+      final statement = buffer.toString().trim();
+      if (statement.isNotEmpty) {
+        statements.add(statement);
+      }
+      buffer.clear();
+      continue;
+    }
+
+    buffer.write(char);
+  }
+
+  final statement = buffer.toString().trim();
+  if (statement.isNotEmpty) {
+    statements.add(statement);
+  }
+  return statements;
+}
+
+String? _readDollarQuoteTag(String sql, int start) {
+  final end = sql.indexOf(r'$', start + 1);
+  if (end == -1) {
+    return null;
+  }
+  final tagName = sql.substring(start + 1, end);
+  final validTag = RegExp(r'^[A-Za-z_][A-Za-z0-9_]*$|^$').hasMatch(tagName);
+  return validTag ? sql.substring(start, end + 1) : null;
+}