Replay a webhook delivery

Re-enqueues one delivery, Stripe-style: creates a FRESH PENDING WebhookDelivery row for the SAME event and endpoint (attempt count 0, next attempt now) that the worker picks up on its normal cycle — the original row is never mutated, so the log keeps the full history. Allowed only when the target delivery is in a terminal state (FAILED, SUCCEEDED or DISABLED — replaying a succeeded delivery is a deliberate resend). Answers 409 delivery_already_pending when ANY delivery for this event+endpoint pair is already PENDING or DELIVERING (no double-enqueue) — which also makes an accidental double-click naturally safe, so this route does not use the Idempotency-Key ledger. The replayed delivery carries the SAME webhook-id signature header as the original (the event id), so consumer-side dedup keeps working. Replaying does NOT re-enable a disabled endpoint — the worker only delivers to enabled endpoints; re-enable via the update route first.