feat: delivery log tab with status filter and manual retry

[cokehill]

Summary

Developers need visibility into what happened to each notification: was it delivered?
How many retries? What was the last error? Can they trigger a retry manually?

Acceptance criteria

• Delivery log table: status badge, event type, subscription ID, attempt count, timestamp
• Filter bar: All | Delivered | Pending | Failed
• "Retry" button visible on failed rows — calls POST /api/deliveries/:id/retry
• Auto-refreshes every 10 seconds via SWR
• Empty state with helpful message
• Pagination or limit to last 100 entries

Notes

Use the useSWR hook with refreshInterval: 10_000.
Subscription ID in table should truncate to 8 chars + ellipsis.