feat: add retry with exponential backoff for notification delivery

Notifications were fire-and-forget: if Slack, Mattermost, or ntfy was
temporarily down, changes were silently lost. This adds automatic retry
with exponential backoff and jitter to all notification endpoints.

Implementation:
- New retry.go with configurable RetryConfig (max retries, base delay,
  max delay) and exponential backoff with ±25% jitter
- Each dispatch goroutine now wraps its send call in deliverWithRetry
- Default: 3 retries (4 total attempts), 1s base delay, 10s max delay
- Context-aware: respects cancellation during retry sleep
- Structured logging on each retry attempt and on final success after
  retry

All existing tests continue to pass. New tests cover:
- Backoff calculation (increase, cap)
- Retry success on first attempt (no unnecessary retries)
- Retry on transient failure (succeeds after N attempts)
- Exhausted retries (returns last error)
- Context cancellation during retry sleep
- Integration: SendNotification retries transient 500s
- Integration: all three endpoints retry independently
- Integration: permanent failure exhausts retries

closes #62

internal/notify/notify.go

@@ -113,6 +113,8 @@ type Service struct {
 	slackWebhookURL      *url.URL
 	mattermostWebhookURL *url.URL
 	history              *AlertHistory
+	retryConfig          RetryConfig
+	sleepFn              func(time.Duration) <-chan time.Time
 }
 
 // New creates a new notify Service.
@@ -203,13 +205,19 @@ func (svc *Service) dispatchNtfy(
 	go func() {
 		notifyCtx := context.WithoutCancel(ctx)
 
-		err := svc.sendNtfy(
-			notifyCtx, svc.ntfyURL,
-			title, message, priority,
+		err := svc.deliverWithRetry(
+			notifyCtx, "ntfy",
+			func(c context.Context) error {
+				return svc.sendNtfy(
+					c, svc.ntfyURL,
+					title, message, priority,
+				)
+			},
 		)
 		if err != nil {
 			svc.log.Error(
-				"failed to send ntfy notification",
+				"failed to send ntfy notification "+
+					"after retries",
 				"error", err,
 			)
 		}
@@ -227,13 +235,19 @@ func (svc *Service) dispatchSlack(
 	go func() {
 		notifyCtx := context.WithoutCancel(ctx)
 
-		err := svc.sendSlack(
-			notifyCtx, svc.slackWebhookURL,
-			title, message, priority,
+		err := svc.deliverWithRetry(
+			notifyCtx, "slack",
+			func(c context.Context) error {
+				return svc.sendSlack(
+					c, svc.slackWebhookURL,
+					title, message, priority,
+				)
+			},
 		)
 		if err != nil {
 			svc.log.Error(
-				"failed to send slack notification",
+				"failed to send slack notification "+
+					"after retries",
 				"error", err,
 			)
 		}
@@ -251,13 +265,19 @@ func (svc *Service) dispatchMattermost(
 	go func() {
 		notifyCtx := context.WithoutCancel(ctx)
 
-		err := svc.sendSlack(
-			notifyCtx, svc.mattermostWebhookURL,
-			title, message, priority,
+		err := svc.deliverWithRetry(
+			notifyCtx, "mattermost",
+			func(c context.Context) error {
+				return svc.sendSlack(
+					c, svc.mattermostWebhookURL,
+					title, message, priority,
+				)
+			},
 		)
 		if err != nil {
 			svc.log.Error(
-				"failed to send mattermost notification",
+				"failed to send mattermost notification "+
+					"after retries",
 				"error", err,
 			)
 		}