Webhook delivery log viewer with retry

[therealjhay]

ProblemThe developers page has a tester but no history of delivered webhook attempts, retry counts, or dead-letter entries.

Solution

• Add app/(merchant)/developers/webhooks/page.tsx showing attempts for the merchant's subscriptions: payload, status code, latency, next retry.
• Add per-attempt "view payload" and "replay" actions.
• Show retry backoff schedule and DLQ entries with manual drain.
• Poll at 10s with hidden-tab pause; render attempt status with StatusBadge vocabulary.

Acceptance Criteria

• A failing delivery shows its retry schedule and a working replay.
• Payload viewer renders formatted JSON with a copy button.
• DLQ drain moves entries to reprocessing and reflects state.
• npm run build passes.

Note for Contributors: Write a clear PR description. Show a 5xx attempt retrying and succeeding.