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>

config/test.exs

@@ -35,3 +35,6 @@ config :phoenix, :plug_init_mode, :runtime
 # Enable helpful, but potentially expensive runtime checks
 config :phoenix_live_view,
   enable_expensive_runtime_checks: true
+
+# Use inline testing mode for Oban
+config :simpleshop_theme, Oban, testing: :inline