add activity log with order timeline and global feed

Single activity_log table powering two views: chronological timeline
on each order detail page (replacing the old fulfilment card) and a
global feed at /admin/activity with tabs, category filters, search,
and pagination. Real-time via PubSub — new entries appear instantly,
nav badge updates across all admin pages.

Instrumented across all event points: Stripe webhooks, order notifier,
submission worker, fulfilment status worker, product sync worker, and
Oban exhausted-job telemetry. Contextual action buttons (retry
submission, retry sync, dismiss) with Oban unique constraints to
prevent double-enqueue. 90-day pruning via cron.

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -80,6 +80,20 @@
                 <.icon name="hero-shopping-bag" class="size-5" /> Orders
               </.link>
             </li>
+            <li>
+              <.link
+                navigate={~p"/admin/activity"}
+                class={admin_nav_active?(@current_path, "/admin/activity")}
+              >
+                <.icon name="hero-bell" class="size-5" /> Activity
+                <span
+                  :if={@attention_count > 0}
+                  class="admin-badge admin-badge-sm admin-badge-warning ml-auto"
+                >
+                  {@attention_count}
+                </span>
+              </.link>
+            </li>
             <li>
               <.link
                 navigate={~p"/admin/products"}