feat: project scaffolding, database schema, config

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/db.php

@@ -0,0 +1,15 @@
+<?php
+require_once __DIR__ . '/config.php';
+
+function getDB(): PDO {
+    static $pdo = null;
+    if ($pdo === null) {
+        $dsn = 'mysql:host=' . DB_HOST . ';dbname=' . DB_NAME . ';charset=utf8mb4';
+        $pdo = new PDO($dsn, DB_USER, DB_PASS, [
+            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
+            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
+            PDO::ATTR_EMULATE_PREPARES => false,
+        ]);
+    }
+    return $pdo;
+}