feat: add oban dependency for background jobs

Add Oban ~> 2.18 with SQLite support (Oban.Engines.Lite) for durable
background job processing. Configure aggressive pruning (60s max_age)
to keep database lean, with a dedicated images queue.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mix.exs

@@ -68,7 +68,8 @@ defmodule SimpleshopTheme.MixProject do
       {:dns_cluster, "~> 0.2.0"},
       {:bandit, "~> 1.5"},
       {:tidewave, "~> 0.5", only: :dev},
-      {:image, "~> 0.54"}
+      {:image, "~> 0.54"},
+      {:oban, "~> 2.18"}
     ]
   end