feat(web): add SQLite schema and connection

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/server/db/index.ts

@@ -0,0 +1,28 @@
+import Database from 'better-sqlite3';
+import path from 'path';
+import fs from 'fs';
+import { fileURLToPath } from 'url';
+import { SCHEMA_SQL } from './schema';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+const DB_PATH = path.resolve(__dirname, '..', '..', 'data', 'meitu-agent.db');
+
+let db: Database.Database;
+
+export function getDb(): Database.Database {
+  if (!db) {
+    const dir = path.dirname(DB_PATH);
+    if (!fs.existsSync(dir)) fs.mkdirSync(dir, { recursive: true });
+    db = new Database(DB_PATH);
+    db.pragma('journal_mode = WAL');
+    db.pragma('foreign_keys = ON');
+  }
+  return db;
+}
+
+export function initDb(): void {
+  const d = getDb();
+  d.exec(SCHEMA_SQL);
+}