Shipped sync retry handling and cached the dashboard

Made GitHub sync recover on its own from transient failures, and stopped the dashboard re-fetching on every visit.

• Added retry handling + a dead-letter path to background sync so activity stops silently going missing (acme/platform-api #482)
• Cached recent activity so the dashboard loads instantly on return instead of cold-fetching (acme/web-app #311)

Sync reliability is invisible when it works and damaging when it doesn't — missing activity quietly corrupts the record the whole product depends on.

Add an alert on dead-letter volume so silent failures surface quickly.