Fix Matrix API notification to use PUT method with transaction ID

- Change HTTP method from POST to PUT for Matrix API v3
- Matrix API requires PUT when transaction ID is included in URL path
- Move transaction ID construction before URL building for clarity
- Fixes "405 Method Not Allowed" error when sending notifications

The Matrix API v3 endpoint structure:
PUT /_matrix/client/v3/rooms/{roomId}/send/{eventType}/{txnId}

Previous error:
POST request was being rejected with 405 Method Not Allowed

Now working:
PUT request successfully sends messages to Matrix rooms

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/alerting.py

@@ -52,7 +52,9 @@ class MatrixNotifier:
     def send_message(self, message: str, msgtype: str = "m.text") -> bool:
         """Send message to Matrix room"""
         try:
-            url = f"{self.homeserver}/_matrix/client/v3/rooms/{self.room_id}/send/m.room.message"
+            # Add transaction ID to prevent duplicates
+            txn_id = datetime.datetime.now().strftime("%Y%m%d_%H%M%S_%f")
+            url = f"{self.homeserver}/_matrix/client/v3/rooms/{self.room_id}/send/m.room.message/{txn_id}"
 
             headers = {
                 "Authorization": f"Bearer {self.access_token}",
@@ -64,11 +66,8 @@ class MatrixNotifier:
                 "body": message
             }
 
-            # Add transaction ID to prevent duplicates
-            txn_id = datetime.datetime.now().strftime("%Y%m%d_%H%M%S_%f")
-            url += f"/{txn_id}"
-
-            response = self.session.post(url, headers=headers, json=data, timeout=10)
+            # Matrix API requires PUT when transaction ID is in the URL path
+            response = self.session.put(url, headers=headers, json=data, timeout=10)
             response.raise_for_status()
 
             logger.info(f"Matrix message sent successfully: {response.json().get('event_id')}")