Add documentation to explain how transport-level and queue-level retries interact:
- Email: MaxRetries (transport) * MaxAttempts (queue) = total SMTP attempts
- Webhooks: RetryCount (transport) * MaxAttempts (queue) = total HTTP attempts
- Example: 3 * 3 = 9 total delivery attempts for a single notification
This clarifies the multiplicative retry behavior and helps operators understand
the actual retry counts when using the persistent queue.
Critical fixes (P0):
- Fix cooldown timing: Mark cooldown only after successful delivery, not before enqueue
- Add os.MkdirAll to queue initialization to prevent silent failures on fresh installs
- Add DNS re-validation at webhook send time to prevent DNS rebinding SSRF attacks
- Add SSRF validation for Apprise HTTP URLs
- Remove secret logging (bot tokens, routing keys) from debug logs
- Implement lastNotified cleanup to prevent unbounded memory growth
- Use shared HTTP client for webhooks to enable TLS connection reuse
- Add fallback to direct sending when queue enqueue fails
- Make queue worker concurrent (5 workers with semaphore) to prevent head-of-line blocking
- Fix webhook rate limiter race condition with separate mutex
- Fix email manager thread safety with mutex on rate limiter
- Fix grouping timer leak by adding stopCleanup signal
- Fix webhook 429 double sleep (use Retry-After OR backoff, not both)
Frontend improvements:
- Add queue/DLQ management API methods (getQueueStats, getDLQ, retryDLQItem, deleteDLQItem)
- Add getNotificationHealth and getWebhookHistory endpoints
- Add Apprise test support to NotificationTestRequest type
Related to notification system audit
