Feature: Recommendation Service

This commit introduces a RecommendationService that is used when indexing products.

The service is utilizing a recommendation rules interface, so many rules can be created and applied during indexing the products

src/Catalog/Recommendations/RandomRule.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Modules\Core\Catalog\Recommendations;
+
+use Illuminate\Support\Collection;
+use Lunar\Models\Product;
+use Modules\Core\Catalog\Contracts\RecommendationRule;
+
+/**
+ * The universal fallback — always returns something as long as the store
+ * has more than one product, since it has no eligibility condition of its
+ * own to come up empty on. Meant to be placed last in
+ * config('catalog.recommendation_rules'), not first: every store using
+ * the default chain gets a real fallback, but one that only kicks in once
+ * more specific rules (same category, same tag, ...) have had a chance.
+ */
+class RandomRule implements RecommendationRule
+{
+    public function recommend(Product $product, int $limit, array $exclude): Collection
+    {
+        return Product::query()
+            ->whereKeyNot($exclude)
+            ->inRandomOrder()
+            ->limit($limit)
+            ->get();
+    }
+}

src/Catalog/Recommendations/SameCategoryRule.php

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+<?php
+
+namespace Modules\Core\Catalog\Recommendations;
+
+use Illuminate\Support\Collection;
+use Lunar\Models\Product;
+use Modules\Core\Catalog\Contracts\RecommendationRule;
+
+/**
+ * Recommends other products sharing at least one of $product's directly-
+ * assigned collections — takes $product's first collection (a product
+ * usually has one primary category; if it has several, the first is as
+ * good a choice as any without a "primary collection" concept to prefer).
+ * Returns nothing if $product has no collection at all, letting the next
+ * rule in the chain (see RecommendationRule's docblock) take over.
+ *
+ * Queries Eloquent directly rather than going through Modules\Core\Catalog\
+ * Services\ProductService — this runs at index time (see
+ * RecommendationRule's docblock), where Meilisearch may be mid-reindex for
+ * this very product and ProductService::list()'s locale-resolution has no
+ * meaningful "current locale" to resolve against anyway.
+ */
+class SameCategoryRule implements RecommendationRule
+{
+    public function recommend(Product $product, int $limit, array $exclude): Collection
+    {
+        $collection = $product->collections->first();
+
+        if ($collection === null) {
+            return collect();
+        }
+
+        return $collection->products()
+            ->whereKeyNot($exclude)
+            ->inRandomOrder()
+            ->limit($limit)
+            ->get();
+    }
+}