Per-Webhook Backoff Strategy

Choose exponential, linear, or fixed backoff strategy and delay parameters per webhook.

Configure the retry delay strategy for each webhook independently. Three strategies: `exponential` (base_delay × 2^attempt, capped at max_delay), `linear` (base_delay × attempt), `fixed` (always base_delay). Resolution order: per-webhook → global Pro setting → plugin default (exponential, 30s base, 3600s cap).