feat: Initial commit for Saqel Platform architecture, backend, Docker, and documentation

backend/app/Models/Lesson.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Models;
+
+use Database\Factories\LessonFactory;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+class Lesson extends Model
+{
+    /** @use HasFactory<LessonFactory> */
+    use HasFactory;
+
+    protected $fillable = ['course_id', 'title', 'order_index', 'video_id', 'duration_seconds', 'transcript'];
+
+    public function course(): BelongsTo
+    {
+        return $this->belongsTo(Course::class);
+    }
+
+    public function concepts(): BelongsToMany
+    {
+        return $this->belongsToMany(Concept::class);
+    }
+
+    public function quizzes(): HasMany
+    {
+        return $this->hasMany(Quiz::class);
+    }
+
+    public function progressFor(int $userId): ?LessonProgress
+    {
+        /** @var LessonProgress|null $progress */
+        $progress = $this->hasOne(LessonProgress::class)
+            ->where('user_id', $userId)
+            ->first();
+
+        return $progress;
+    }
+}