Add proactive webhook health checks and actionable operational alerts

Problem/Motivation

Existing diagnostics show aggregate counters and credential readiness, but operators must inspect them manually. A stale webhook, expiring rotation overlap or growing retention backlog can otherwise go unnoticed.

Proposed resolution

Extend existing diagnostics with configurable health evaluation and Drupal status-report/Drush output. Provide an optional notification integration without requiring this module to send mail.

Acceptance criteria

• Expose last successful intake, configured-source health where trustworthy, retention backlog and credential-rotation readiness without recipient or secret labels.
• Treat absence of traffic as unknown unless an operator has configured an expected activity window; quiet sites must not be reported broken.
• Allow thresholds, cooldowns and recovery notifications; deduplicate repeated alerts.
• Offer machine-readable output and test time boundaries, no-traffic installs, transient failures and notification delivery failures.
• When optional provider status is available, distinguish provider-paused delivery from local silence and never claim local counters prove endpoint reachability.

Scope and dependencies

Optional follow-up after stable release; not a blocker for #3621224: Complete the 1.0.0 stable release verification gates. Builds on #3621131: Add a no-send policy preview and honest effective-status diagnostics; the completed original issue remains fixed.