Show related packages based on description embeddings

Package pages get a "Related packages" section: a nearest neighbor
query against the package's own stored embedding (no external service
call), excluding the package itself. Packages without an embedding
render no section.

app/models/package.rb

@@ -112,6 +112,16 @@ class Package < ApplicationRecord
     text.gsub(/[\\%_]/) { |char| "\\#{char}" }
   end
 
+  # Packages semantically similar to this one, based on the stored
+  # description embedding. Returns an empty relation for packages that
+  # have no embedding (yet).
+  def related_packages(limit: 6)
+    return Package.none unless embedding
+
+    Package.nearest_to_vector(embedding, limit: limit + 1)
+           .where.not(id: id).limit(limit)
+  end
+
   # Check whether the given text shares at least one lexeme with any
   # package name/description in the database. Queries without such an
   # overlap (gibberish, keyboard mash, unsupported languages) can
@@ -209,3 +219,4 @@ class Package < ApplicationRecord
     end
   end
 end
+# rubocop:enable Metrics/ClassLength