Expand IGDB data: collections, game modes, themes, perspectives, videos, websites, age ratings, similar games; add backfill command

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/Theme.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+
+class Theme extends Model
+{
+    protected $fillable = ['igdb_id', 'name', 'slug'];
+
+    public function games(): BelongsToMany
+    {
+        return $this->belongsToMany(Game::class, 'game_theme');
+    }
+}