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

backend/app/Models/Concept.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Models;
+
+use Database\Factories\ConceptFactory;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+class Concept extends Model
+{
+    /** @use HasFactory<ConceptFactory> */
+    use HasFactory;
+
+    protected $fillable = ['subject_id', 'name', 'remedial_lesson_id'];
+
+    public function subject(): BelongsTo
+    {
+        return $this->belongsTo(Subject::class);
+    }
+
+    public function remedialLesson(): BelongsTo
+    {
+        return $this->belongsTo(Lesson::class, 'remedial_lesson_id');
+    }
+
+    public function questions(): HasMany
+    {
+        return $this->hasMany(Question::class);
+    }
+}