feat: add retry with exponential backoff for notification delivery (#87)

## Summary

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.

## Changes

### New file: `internal/notify/retry.go`
- `RetryConfig` struct with configurable max retries, base delay, max delay
- `backoff()` computes delay as `BaseDelay * 2^attempt`, capped at `MaxDelay`, with ±25% jitter
- `deliverWithRetry()` wraps any send function with the retry loop
- Defaults: 3 retries (4 total attempts), 1s base delay, 10s max delay
- Context-aware: respects cancellation during retry sleep
- Injectable `sleepFn` for test determinism

### Modified: `internal/notify/notify.go`
- Added `retryConfig` and `sleepFn` fields to `Service`
- Updated `dispatchNtfy`, `dispatchSlack`, `dispatchMattermost` to wrap sends in `deliverWithRetry`
- Structured logging: warns on each retry, logs error only after all retries exhausted, logs info on success after retry

### Modified: `internal/notify/export_test.go`
- Added test helpers: `SetRetryConfig`, `SetSleepFunc`, `DeliverWithRetry`, `BackoffDuration`

### New file: `internal/notify/retry_test.go`
- Backoff calculation tests (exponential increase, max cap with jitter)
- `deliverWithRetry` unit tests: first-attempt success, transient failure recovery, exhausted retries, context cancellation
- Integration tests via `SendNotification`: transient failure retries, all-endpoints retry independently, permanent failure exhausts retries

## Verification
- `make fmt` ✅
- `make check` (format + lint + tests + build) ✅
- `docker build .` ✅
- All existing tests continue to pass unchanged
- No DNS client mocking — notification tests use `httptest` servers

closes #62

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #87
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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)
+}