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/components/layouts/admin.html.heex

@@ -208,6 +208,11 @@
               <.icon name="hero-bug-ant" class="size-5" /> Errors
             </.link>
           </li>
+          <li>
+            <.link href={~p"/admin/oban"}>
+              <.icon name="hero-queue-list" class="size-5" /> Jobs
+            </.link>
+          </li>
           <li>
             <.link href={~p"/users/log-out"} method="delete">
               <.icon name="hero-arrow-right-start-on-rectangle" class="size-5" /> Log out