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>

README.md

@@ -231,6 +231,7 @@ the following precedence (highest to lowest):
 | `DNSWATCHER_MAINTENANCE_MODE`   | Enable maintenance mode                    | `false`     |
 | `DNSWATCHER_METRICS_USERNAME`   | Basic auth username for /metrics           | `""`        |
 | `DNSWATCHER_METRICS_PASSWORD`   | Basic auth password for /metrics           | `""`        |
+| `DNSWATCHER_SEND_TEST_NOTIFICATION` | Send a test notification after first scan completes | `false` |
 
 **`DNSWATCHER_TARGETS` is required.** dnswatcher will refuse to start if no
 monitoring targets are configured. A monitoring daemon with nothing to monitor
@@ -248,6 +249,7 @@ DNSWATCHER_TARGETS=example.com,example.org,www.example.com,api.example.com,mail.
 DNSWATCHER_SLACK_WEBHOOK=https://hooks.slack.com/services/T.../B.../xxx
 DNSWATCHER_MATTERMOST_WEBHOOK=https://mattermost.example.com/hooks/xxx
 DNSWATCHER_NTFY_TOPIC=https://ntfy.sh/my-dns-alerts
+DNSWATCHER_SEND_TEST_NOTIFICATION=true
 ```
 
 ---
@@ -380,6 +382,7 @@ docker run -d \
   -v dnswatcher-data:/var/lib/dnswatcher \
   -e DNSWATCHER_TARGETS=example.com,www.example.com \
   -e DNSWATCHER_NTFY_TOPIC=https://ntfy.sh/my-alerts \
+  -e DNSWATCHER_SEND_TEST_NOTIFICATION=true \
   dnswatcher
 ```