feat: Implement alert creation service with FCM notifications

- Add AlertCreationService to handle alert creation and notifications.
- Implement idempotency key support to prevent duplicate alerts.
- Create alert notification service for sending FCM messages.
- Add tests for alert creation and notification services.
- Document FCM notification flow and alert creation process.

docs/openapi-draft.yaml

@@ -1360,6 +1360,15 @@ paths:
       security:
         - BearerAuth: []
         - ApiKeyAuth: []
+      parameters:
+        - in: header
+          name: Idempotency-Key
+          required: false
+          description: Stable key for retrying one logical alert creation without duplicate alerts or pushes
+          schema:
+            type: string
+            minLength: 1
+            maxLength: 255
       requestBody:
         required: true
         content:
@@ -1379,6 +1388,12 @@ paths:
           $ref: '#/components/responses/Unauthorized'
         '403':
           $ref: '#/components/responses/Forbidden'
+        '409':
+          description: Idempotency-Key was already used with a different request payload
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
         '429':
           $ref: '#/components/responses/TooManyRequests'
         '500':