feat: PHP API foundation - router, config, helpers, SM-2 algorithm

edu/api/config.php

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+<?php
+$db_host = getenv('EDU_DB_HOST') ?: 'postgres';
+$db_port = getenv('EDU_DB_PORT') ?: '5432';
+$db_name = getenv('EDU_DB_NAME') ?: 'edu';
+$db_user = getenv('EDU_DB_USER') ?: 'postgres';
+$db_pass = getenv('EDU_DB_PASS') ?: '';
+
+try {
+    $pdo = new PDO(
+        "pgsql:host={$db_host};port={$db_port};dbname={$db_name}",
+        $db_user, $db_pass,
+        [
+            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
+            PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
+            PDO::ATTR_EMULATE_PREPARES => false
+        ]
+    );
+} catch (PDOException $e) {
+    http_response_code(500);
+    echo json_encode(['error' => 'Datenbankverbindung fehlgeschlagen']);
+    exit;
+}
+
+ini_set('session.cookie_httponly', 1);
+ini_set('session.cookie_secure', 1);
+ini_set('session.cookie_samesite', 'Strict');
+ini_set('session.use_strict_mode', 1);
+session_start();
+
+define('CONTENT_DIR', __DIR__ . '/../content');

edu/api/helpers.php

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+<?php
+header('Content-Type: application/json; charset=utf-8');
+
+function json_ok($data) {
+    echo json_encode($data, JSON_UNESCAPED_UNICODE);
+    exit;
+}
+
+function json_error($message, $code = 400) {
+    http_response_code($code);
+    echo json_encode(['error' => $message], JSON_UNESCAPED_UNICODE);
+    exit;
+}
+
+function require_auth() {
+    if (empty($_SESSION['user_id'])) {
+        json_error('Nicht angemeldet', 401);
+    }
+    return $_SESSION['user_id'];
+}
+
+function require_admin() {
+    require_auth();
+    if (empty($_SESSION['is_admin'])) {
+        json_error('Keine Admin-Berechtigung', 403);
+    }
+    return $_SESSION['user_id'];
+}
+
+function get_method() {
+    return $_SERVER['REQUEST_METHOD'];
+}
+
+function get_json_body() {
+    $body = file_get_contents('php://input');
+    return json_decode($body, true) ?: [];
+}
+
+function get_param($name, $default = null) {
+    return $_GET[$name] ?? $default;
+}

edu/api/index.php

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+<?php
+require_once __DIR__ . '/config.php';
+require_once __DIR__ . '/helpers.php';
+
+// Parse: /api/decks/omnis-commands -> ['decks', 'omnis-commands']
+$request_uri = $_SERVER['REQUEST_URI'];
+$path = parse_url($request_uri, PHP_URL_PATH);
+$path = preg_replace('#^/api/?#', '', $path);
+$path = trim($path, '/');
+$segments = $path ? explode('/', $path) : [];
+$resource = $segments[0] ?? '';
+
+switch ($resource) {
+    case 'login':
+    case 'logout':
+    case 'me':
+        require __DIR__ . '/auth.php';
+        break;
+    case 'decks':
+        require __DIR__ . '/decks.php';
+        break;
+    case 'cards':
+        require __DIR__ . '/cards.php';
+        break;
+    case 'tutorials':
+        require __DIR__ . '/tutorials.php';
+        break;
+    case 'quiz':
+        require __DIR__ . '/quiz.php';
+        break;
+    case 'cheatsheets':
+        require __DIR__ . '/cheatsheets.php';
+        break;
+    case 'dashboard':
+        require __DIR__ . '/dashboard.php';
+        break;
+    case 'admin':
+        require __DIR__ . '/admin.php';
+        break;
+    default:
+        json_error('Unbekannter Endpunkt', 404);
+}

edu/api/sm2.php

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+<?php
+/**
+ * SM-2 Spaced Repetition Algorithm
+ * Ratings: easy=5, medium=3, hard=2, wrong=0
+ */
+function sm2_calculate($rating_name, $current_ease, $current_interval, $current_reps) {
+    $rating_map = ['wrong' => 0, 'hard' => 2, 'medium' => 3, 'easy' => 5];
+    $q = $rating_map[$rating_name] ?? 3;
+
+    $ease = $current_ease;
+    $interval = $current_interval;
+    $reps = $current_reps;
+
+    if ($q < 3) {
+        $reps = 0;
+        $interval = 1;
+    } else {
+        if ($reps === 0) {
+            $interval = 1;
+        } elseif ($reps === 1) {
+            $interval = 3;
+        } else {
+            $interval = (int) round($interval * $ease);
+        }
+        $reps++;
+    }
+
+    $ease = $ease + 0.1 - (5 - $q) * (0.08 + (5 - $q) * 0.02);
+    if ($ease < 1.3) $ease = 1.3;
+
+    $next_review = date('Y-m-d', strtotime("+{$interval} days"));
+
+    return [
+        'ease_factor' => round($ease, 2),
+        'interval_days' => $interval,
+        'repetitions' => $reps,
+        'next_review' => $next_review
+    ];
+}