Fix: Mattermost/Slack Webhook Support

Root Cause: The generic webhook sent a JSON payload with custom field names plus timestamp and source fields. Mattermost/Slack webhooks only accept {"text": "..."}
format and reject unknown fields with HTTP 400.

Users just need to select "Slack / Mattermost" from the Payload Format dropdown when configuring their Mattermost webhook.

Closes #133

backend/app/services/notification_service.py

@@ -356,22 +356,33 @@ class NotificationService:
             return False, f"HTTP {response.status_code}: {response.text[:200]}"
 
     async def _send_webhook(self, config: dict, title: str, message: str) -> tuple[bool, str]:
-        """Send notification via generic webhook (POST JSON)."""
+        """Send notification via generic webhook (POST JSON).
+
+        Supports two payload formats:
+        - generic: Custom field names with timestamp/source metadata
+        - slack: Slack/Mattermost compatible format (just {"text": "..."})
+        """
         webhook_url = config.get("webhook_url", "").strip()
         auth_header = config.get("auth_header", "").strip()
-        custom_field_title = config.get("field_title", "title").strip() or "title"
-        custom_field_message = config.get("field_message", "message").strip() or "message"
+        payload_format = config.get("payload_format", "generic").strip()
 
         if not webhook_url:
             return False, "Webhook URL is required"
 
-        # Build payload with custom field names
-        data = {
-            custom_field_title: title,
-            custom_field_message: message,
-            "timestamp": datetime.now().isoformat(),
-            "source": "Bambuddy",
-        }
+        # Build payload based on format
+        if payload_format == "slack":
+            # Slack/Mattermost format - just text field
+            data = {"text": f"*{title}*\n{message}"}
+        else:
+            # Generic format with custom field names
+            custom_field_title = config.get("field_title", "title").strip() or "title"
+            custom_field_message = config.get("field_message", "message").strip() or "message"
+            data = {
+                custom_field_title: title,
+                custom_field_message: message,
+                "timestamp": datetime.now().isoformat(),
+                "source": "Bambuddy",
+            }
 
         headers = {"Content-Type": "application/json"}
         if auth_header:

backend/tests/unit/services/test_notification_service.py

@@ -557,6 +557,38 @@ class TestNotificationProviderTypes:
             assert success is False
             assert "Connection failed" in message or "error" in message.lower()
 
+    @pytest.mark.asyncio
+    async def test_webhook_slack_format_sends_text_only(self, service):
+        """Verify Slack/Mattermost format sends only text field."""
+        config = {
+            "webhook_url": "http://mattermost.local/hooks/abc123",
+            "payload_format": "slack",
+        }
+
+        mock_response = MagicMock()
+        mock_response.status_code = 200
+
+        mock_client = AsyncMock()
+        mock_client.post = AsyncMock(return_value=mock_response)
+
+        with patch.object(service, "_get_client", new_callable=AsyncMock) as mock_get_client:
+            mock_get_client.return_value = mock_client
+
+            success, message = await service._send_webhook(config, "Test Title", "Test Message")
+
+            assert success is True
+            mock_client.post.assert_called_once()
+
+            # Verify payload format is Slack-compatible
+            call_args = mock_client.post.call_args
+            payload = call_args.kwargs.get("json") or call_args[1].get("json")
+            assert "text" in payload
+            assert "*Test Title*" in payload["text"]
+            assert "Test Message" in payload["text"]
+            # Should NOT have generic fields
+            assert "timestamp" not in payload
+            assert "source" not in payload
+
 
 class TestNotificationVariableFallbacks:
     """Tests for notification variable fallback values."""