feat: add GET /api/v1/programs/:id/webhook-events audit endpoint (Closes #18)

[ZacLou]

Closes #18

Changes

Adds GET /api/v1/programs/:id/webhook-events audit endpoint with cursor pagination:

• Endpoint: GET /api/v1/programs/{id}/webhook-events?limit=20&cursor=<timestamp>
• Response: { "items": [...], "next_cursor": "<ts>", "has_more": bool }
• Filtering: Matches webhook events to program via github_repo join
• 404: Returns "program not found" when program ID doesn't exist
• Fields: id, delivery_id, event_type, github_repo, pr_number, status, error_message, received_at

Files changed

• crates/api/src/routes/mod.rs — Added route, handler (list_webhook_events), WebhookEventRow, and WebhookEventRecord types

Use case

Audit trail for debugging webhook processing — see which deliveries were processed, ignored, or failed for a given program.