[BUG] Notifcations are sent multiple times if there are multiple API Deployments

**Describe the bug**
If Peekaping is deployed with multiple instances of the API notifications are sent by each instance of the API

**Database**
- postgresql

**To Reproduce**
Steps to reproduce the behavior:
1. Setup a Notification Channel
2. Have multiple Instances of the API
3. Force a downtime or uptime (create a new monitor), needs a notification channel selected
4. See multiple notifications about the event, this is always equivalent to the count of the API instances

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
API Instance 1
```
{"level":"info","ts":1765296683.557561,"logger":"[monitor-event-listener]","caller":"monitor/listener.go:49","msg":"Monitor status changed event received for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf, new status: 0","svc":"peekaping:api"}
{"level":"info","ts":1765296683.5571902,"caller":"notification_channel/listener.go:89","msg":"Notification event received for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf","svc":"peekaping:api"}
{"level":"info","ts":1765296683.597729,"caller":"notification_channel/listener.go:100","msg":"Monitor notification: 63463c04-2892-4026-a612-0d10586bf64d","svc":"peekaping:api"}
{"level":"info","ts":1765296683.6793463,"caller":"providers/ntfy.go:163","msg":"Sending NTFY notification to https://ntfy.example.com with topic server-alerts","svc":"peekaping:api"}
{"level":"info","ts":1765296683.7353666,"caller":"providers/ntfy.go:181","msg":"NTFY notification sent successfully to https://ntfy.example.com","svc":"peekaping:api"}
{"level":"info","ts":1765296683.7354386,"caller":"notification_channel/listener.go:141","msg":"Notification sent to: NTFY for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf","svc":"peekaping:api"}
{"level":"info","ts":1765296683.8284128,"logger":"[monitor-event-listener]","caller":"monitor/listener.go:80","msg":"Successfully updated monitor dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf status from 1 to 0","svc":"peekaping:api"}
```

API Instance 2
```
{"level":"info","ts":1765296683.5576427,"logger":"[monitor-event-listener]","caller":"monitor/listener.go:49","msg":"Monitor status changed event received for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf, new status: 0","svc":"peekaping:api"}
{"level":"info","ts":1765296683.557448,"caller":"notification_channel/listener.go:89","msg":"Notification event received for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf","svc":"peekaping:api"}
{"level":"info","ts":1765296683.61741,"caller":"notification_channel/listener.go:100","msg":"Monitor notification: 63463c04-2892-4026-a612-0d10586bf64d","svc":"peekaping:api"}
{"level":"info","ts":1765296683.772907,"caller":"providers/ntfy.go:163","msg":"Sending NTFY notification to https://ntfy.example.com with topic server-alerts","svc":"peekaping:api"}
{"level":"info","ts":1765296683.813862,"logger":"[monitor-event-listener]","caller":"monitor/listener.go:80","msg":"Successfully updated monitor dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf status from 1 to 0","svc":"peekaping:api"}
{"level":"info","ts":1765296683.8185625,"caller":"providers/ntfy.go:181","msg":"NTFY notification sent successfully to https://ntfy.example.com","svc":"peekaping:api"}
{"level":"info","ts":1765296683.818652,"caller":"notification_channel/listener.go:141","msg":"Notification sent to: NTFY for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf","svc":"peekaping:api"}
```

API Instance 3
```
{"level":"info","ts":1765296683.558118,"logger":"[monitor-event-listener]","caller":"monitor/listener.go:49","msg":"Monitor status changed event received for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf, new status: 0","svc":"peekaping:api"}
{"level":"info","ts":1765296683.5587647,"caller":"notification_channel/listener.go:89","msg":"Notification event received for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf","svc":"peekaping:api"}
{"level":"info","ts":1765296683.5905206,"caller":"notification_channel/listener.go:100","msg":"Monitor notification: 63463c04-2892-4026-a612-0d10586bf64d","svc":"peekaping:api"}
{"level":"info","ts":1765296683.6671205,"caller":"providers/ntfy.go:163","msg":"Sending NTFY notification to https://ntfy.example.com with topic server-alerts","svc":"peekaping:api"}
{"level":"info","ts":1765296683.7791312,"logger":"[monitor-event-listener]","caller":"monitor/listener.go:80","msg":"Successfully updated monitor dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf status from 1 to 0","svc":"peekaping:api"}
{"level":"info","ts":1765296683.8315582,"caller":"providers/ntfy.go:181","msg":"NTFY notification sent successfully to https://ntfy.example.com","svc":"peekaping:api"}
{"level":"info","ts":1765296683.8316314,"caller":"notification_channel/listener.go:141","msg":"Notification sent to: NTFY for monitor: dc6b3bbd-2adc-4aa1-b303-87ca2285eaaf","svc":"peekaping:api"}
```

**Additional context**
Doing a test notification does not help, as there it seems as the API Instance handling the Web request immediately sends the notification not going through the same notification event trigger as status changes

The bug is in the notification event handling across multiple API instances. Start by examining the listener code in notification_channel/listener.go and monitor/listener.go to see how events are consumed. Look for a mechanism to deduplicate events, possibly using a distributed lock or a database flag to mark a notification as sent. Check if the project uses a message queue or a shared state in PostgreSQL. Running the tests for the notification system will help verify the fix.

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