ML-170: add enrichment fields to collection API

Add MusicLibrary.Collection.Enrichment module with batch scrobble, artist
country, and selected release enrichment from existing tables.

Wire enrichment into all four collection API endpoints (index, latest,
random, on_this_day). New JSON fields: scrobble_count, last_listened_at,
artist_country, selected_release.

Uses 3 fixed-count batch queries regardless of page size — no N+1 risk.
No schema changes or migrations required.

test/music_library_web/controllers/collection_controller_test.exs

@@ -327,6 +327,17 @@ defmodule MusicLibraryWeb.CollectionControllerTest do
         "large" => asset_url(record.cover_hash, 1000),
         "medium" => asset_url(record.cover_hash, 400),
         "small" => asset_url(record.cover_hash, 80)
+      },
+      "scrobble_count" => 0,
+      "last_listened_at" => nil,
+      "artist_country" => nil,
+      "selected_release" => %{
+        "catalog_number" => "",
+        "country" => "GB",
+        "date" => "2004-05-03",
+        "disambiguation" => "",
+        "format" => "multi",
+        "packaging" => "Jewel Case"
       }
     }
   end