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/router.ex

@@ -148,6 +148,7 @@ defmodule BerrypodWeb.Router do
       live "/analytics", Admin.Analytics, :index
       live "/orders", Admin.Orders, :index
       live "/orders/:id", Admin.OrderShow, :show
+      live "/activity", Admin.Activity, :index
       live "/products", Admin.Products, :index
       live "/products/:id", Admin.ProductShow, :show
       live "/providers", Admin.Providers.Index, :index