Document layered retry semantics (P2 documentation)

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.

internal/notifications/email_enhanced.go

@@ -37,6 +37,11 @@ func NewEnhancedEmailManager(config EmailProviderConfig) *EnhancedEmailManager {
 }
 
 // SendEmailWithRetry sends email with retry logic
+// Note: When used with the persistent queue, retry behavior is layered:
+// - Transport retries (this function): up to MaxRetries attempts with RetryDelay between
+// - Queue retries: up to MaxAttempts (default 3) with exponential backoff
+// Total attempts = MaxRetries * MaxAttempts (e.g., 3 * 3 = 9 SMTP calls for a single notification)
+// This ensures delivery even during transient failures at either layer.
 func (e *EnhancedEmailManager) SendEmailWithRetry(subject, htmlBody, textBody string) error {
 	var lastErr error
 

internal/notifications/webhook_enhanced.go

@@ -194,6 +194,11 @@ func (n *NotificationManager) shouldSendWebhook(webhook EnhancedWebhookConfig, a
 }
 
 // sendWebhookWithRetry implements exponential backoff retry with enhanced error tracking
+// Note: When used with the persistent queue, retry behavior is layered:
+// - Transport retries (this function): up to RetryCount attempts with exponential backoff
+// - Queue retries: up to MaxAttempts (default 3) with exponential backoff
+// Total attempts = RetryCount * MaxAttempts (e.g., 3 * 3 = 9 HTTP calls for a single notification)
+// This ensures delivery even during transient failures at either layer.
 func (n *NotificationManager) sendWebhookWithRetry(webhook EnhancedWebhookConfig, payload []byte) error {
 	maxRetries := webhook.RetryCount
 	if maxRetries <= 0 {