feat: 添加 Vercel Postgres 数据库支持

- 安装 @vercel/postgres 和 pg 依赖
- 创建 PostgresAdapter 适配器，兼容 D1 接口
- 创建 PostgresStorage 类，实现完整的 IStorage 接口
- 添加 Postgres 数据库初始化脚本和 schema
- 更 next.config.js 排除 Postgres 模块的客户端打包
- 添加 VERCEL_DEPLOYMENT.md 部署指南
- 支持 Vercel serverless 环境部署

注意事项：观影室功能在 Vercel 上不可用（需要 WebSocket 支持）

scripts/init-postgres.js

@@ -0,0 +1,62 @@
+/**
+ * Vercel Postgres 数据库初始化脚本
+ *
+ * 创建数据库表结构并初始化默认管理员用户
+ */
+
+const { sql } = require('@vercel/postgres');
+const crypto = require('crypto');
+
+// SHA-256 加密密码
+function hashPassword(password) {
+  return crypto.createHash('sha256').update(password).digest('hex');
+}
+
+console.log('📦 Initializing Vercel Postgres database...');
+
+// 读取迁移脚本
+const fs = require('fs');
+const path = require('path');
+
+const sqlPath = path.join(__dirname, '../migrations/postgres/001_initial_schema.sql');
+if (!fs.existsSync(sqlPath)) {
+  console.error('❌ Migration file not found:', sqlPath);
+  process.exit(1);
+}
+
+const schemaSql = fs.readFileSync(sqlPath, 'utf8');
+
+async function init() {
+  try {
+    // 执行 schema 创建
+    console.log('🔧 Creating database schema...');
+    await sql.unsafe(schemaSql);
+    console.log('✅ Database schema created successfully!');
+
+    // 创建默认管理员用户
+    const username = process.env.USERNAME || 'admin';
+    const password = process.env.PASSWORD || '123456789';
+    const passwordHash = hashPassword(password);
+
+    console.log('👤 Creating default admin user...');
+    await sql`
+      INSERT INTO users (username, password_hash, role, created_at, playrecord_migrated, favorite_migrated, skip_migrated)
+      VALUES (${username}, ${passwordHash}, 'owner', ${Date.now()}, 1, 1, 1)
+      ON CONFLICT (username) DO NOTHING
+    `;
+    console.log(`✅ Default admin user created: ${username}`);
+
+    console.log('');
+    console.log('🎉 Vercel Postgres database initialized successfully!');
+    console.log('');
+    console.log('Next steps:');
+    console.log('1. Set NEXT_PUBLIC_STORAGE_TYPE=postgres in .env');
+    console.log('2. Set POSTGRES_URL environment variable');
+    console.log('3. Run: npm run dev');
+  } catch (err) {
+    console.error('❌ Initialization failed:', err);
+    process.exit(1);
+  }
+}
+
+init();