Add Quiz feature: 10-question quizzes with progressive scoring (max 40 pts)

- Quizzes table with questions, answer options, attempts, answers
- Question types: multiple_choice, exclusion, true_false, free_text
- Progressive scoring: [1,1,2,2,3,3,4,6,8,10] = max 40 per quiz
- Alpine.js countdown timer per question with auto-submit on timeout
- Admin: CRUD for quizzes + per-question editor, JSON export/import
- Child: quiz overview with best scores, question view, result breakdown
- Nav: Quiz link in child header and admin sidebar

app/Models/Quiz.php

@@ -0,0 +1,10 @@
+<?php
+namespace App\Models;
+use Illuminate\Database\Eloquent\Model;
+class Quiz extends Model {
+    protected $fillable = ['subject_id','title','description','active'];
+    protected $casts    = ['active' => 'boolean'];
+    public function subject()   { return $this->belongsTo(Subject::class); }
+    public function questions() { return $this->hasMany(QuizQuestion::class)->orderBy('sort_order'); }
+    public function attempts()  { return $this->hasMany(QuizAttempt::class); }
+}