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/export_test.go

@@ -6,6 +6,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/url"
+	"time"
 )
 
 // NtfyPriority exports ntfyPriority for testing.
@@ -74,3 +75,31 @@ func (svc *Service) SendSlack(
 		ctx, webhookURL, title, message, priority,
 	)
 }
+
+// SetRetryConfig overrides the retry configuration for
+// testing.
+func (svc *Service) SetRetryConfig(cfg RetryConfig) {
+	svc.retryConfig = cfg
+}
+
+// SetSleepFunc overrides the sleep function so tests can
+// eliminate real delays.
+func (svc *Service) SetSleepFunc(
+	fn func(time.Duration) <-chan time.Time,
+) {
+	svc.sleepFn = fn
+}
+
+// DeliverWithRetry exports deliverWithRetry for testing.
+func (svc *Service) DeliverWithRetry(
+	ctx context.Context,
+	endpoint string,
+	fn func(context.Context) error,
+) error {
+	return svc.deliverWithRetry(ctx, endpoint, fn)
+}
+
+// BackoffDuration exports RetryConfig.backoff for testing.
+func (rc RetryConfig) BackoffDuration(attempt int) time.Duration {
+	return rc.defaults().backoff(attempt)
+}