feat: add DNSWATCHER_SEND_TEST_NOTIFICATION env var (#85)

When set to a truthy value, sends a startup status notification to all configured notification channels after the first full scan completes on application startup. The notification is clearly an all-ok/success message showing the number of monitored domains, hostnames, ports, and certificates.

Changes:
- Added `SendTestNotification` config field reading `DNSWATCHER_SEND_TEST_NOTIFICATION`
- Added `maybeSendTestNotification()` in watcher, called after initial `RunOnce` in `Run`
- Added 3 watcher tests (enabled via Run, enabled via RunOnce alone, disabled)
- Added config tests for the new field
- Updated README: env var table, example .env, Docker example

Closes #84

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #85
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/watcher/watcher.go

@@ -129,6 +129,7 @@ func (w *Watcher) Run(ctx context.Context) {
 	)
 
 	w.RunOnce(ctx)
+	w.maybeSendTestNotification(ctx)
 
 	dnsTicker := time.NewTicker(w.config.DNSInterval)
 	tlsTicker := time.NewTicker(w.config.TLSInterval)
@@ -854,6 +855,38 @@ func (w *Watcher) saveState() {
 	}
 }
 
+// maybeSendTestNotification sends a startup status notification
+// after the first full scan completes, if SEND_TEST_NOTIFICATION
+// is enabled. The message is clearly informational ("all ok")
+// and not an error or anomaly alert.
+func (w *Watcher) maybeSendTestNotification(ctx context.Context) {
+	if !w.config.SendTestNotification {
+		return
+	}
+
+	snap := w.state.GetSnapshot()
+
+	msg := fmt.Sprintf(
+		"dnswatcher has started and completed its initial scan.\n"+
+			"Monitoring %d domain(s) and %d hostname(s).\n"+
+			"Tracking %d port endpoint(s) and %d TLS certificate(s).\n"+
+			"All notification channels are working.",
+		len(snap.Domains),
+		len(snap.Hostnames),
+		len(snap.Ports),
+		len(snap.Certificates),
+	)
+
+	w.log.Info("sending startup test notification")
+
+	w.notify.SendNotification(
+		ctx,
+		"✅ dnswatcher startup complete",
+		msg,
+		"success",
+	)
+}
+
 // --- Utility functions ---
 
 func toSet(items []string) map[string]bool {