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

edu/api/sm2.php

@@ -0,0 +1,39 @@
+<?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
+    ];
+}