GitLab Webhooks get smarter with self-healing capabilities

Introduces self-healing capabilities for GitLab webhooks, where failing webhooks will be temporarily disabled with an increasing backoff period (up to 1 day) instead of being permanently disabled after multiple 4xx errors. All error types (4xx, 5xx, network) will be treated the same. Webhooks will be permanently disabled after 40 successive failures. Existing permanently disabled webhooks will be migrated to this new temporary disablement state.