add Oban Web dashboard at /admin/oban

Bump oban to 2.19+, add oban_web for real-time job monitoring.
Mounted behind admin auth with sidebar link under "Jobs".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/oban_resolver.ex

@@ -0,0 +1,9 @@
+defmodule BerrypodWeb.ObanResolver do
+  @behaviour Oban.Web.Resolver
+
+  @impl true
+  def resolve_user(conn), do: conn.assigns[:current_scope]
+
+  @impl true
+  def resolve_access(_user), do: :all
+end