feat: 添加 Flyway V2 迁移，自动初始化默认管理员账号

- 首次部署自动创建 admin@par.local (trust_level=2)
- 已有账号自动提权（member→admin）
- 幂等安全，重复执行无副作用
- 使用项目 PasswordUtil 生成 bcrypt 哈希，无外部依赖

par-core/src/main/java/com/par/core/db/migration/V2__InitAdmin.java

@@ -0,0 +1,60 @@
+package com.par.core.db.migration;
+
+import com.par.common.util.PasswordUtil;
+import org.flywaydb.core.api.migration.BaseJavaMigration;
+import org.flywaydb.core.api.migration.Context;
+
+import java.sql.PreparedStatement;
+import java.sql.ResultSet;
+
+/**
+ * Flyway V2 迁移：初始化默认管理员账号
+ * <p>
+ * 首次部署时自动创建管理员，已有账号则跳过（幂等）。
+ * 默认密码部署后应立即修改。
+ */
+public class V2__InitAdmin extends BaseJavaMigration {
+
+    private static final String ADMIN_EMAIL = "admin@par.local";
+    private static final String ADMIN_PASSWORD = "Admin@123456";
+    private static final String ADMIN_DISPLAY = "Administrator";
+
+    @Override
+    public void migrate(Context context) throws Exception {
+        // 幂等检查：查询管理员账号是否已存在
+        try (PreparedStatement check = context.getConnection().prepareStatement(
+                "SELECT id, trust_level FROM accounts WHERE email = ?")) {
+            check.setString(1, ADMIN_EMAIL);
+            try (ResultSet rs = check.executeQuery()) {
+                if (rs.next()) {
+                    // 账号已存在，若信任等级不足则自动提权
+                    long id = rs.getLong("id");
+                    int level = rs.getInt("trust_level");
+                    if (level < 2) {
+                        try (PreparedStatement promote = context.getConnection().prepareStatement(
+                                "UPDATE accounts SET trust_level = 2 WHERE id = ?")) {
+                            promote.setLong(1, id);
+                            promote.executeUpdate();
+                        }
+                        System.out.println("[Flyway V2] Account " + ADMIN_EMAIL + " promoted to admin");
+                    }
+                    return;
+                }
+            }
+        }
+
+        // 账号不存在，创建默认管理员
+        String hash = PasswordUtil.encode(ADMIN_PASSWORD);
+        try (PreparedStatement insert = context.getConnection().prepareStatement(
+                "INSERT INTO accounts (email, password_hash, display_name, trust_level, is_active) " +
+                        "VALUES (?, ?, ?, ?, ?)")) {
+            insert.setString(1, ADMIN_EMAIL);
+            insert.setString(2, hash);
+            insert.setString(3, ADMIN_DISPLAY);
+            insert.setInt(4, 2);   // trust_level: 2 = ADMIN
+            insert.setBoolean(5, true);
+            insert.executeUpdate();
+        }
+        System.out.println("[Flyway V2] Default admin created: " + ADMIN_EMAIL);
+    }
+}