Fix external camera not used for snapshot + stream dropping (#325)

The snapshot endpoint always used the internal printer camera even when
an external camera was configured. Now checks for external camera first,
matching the stream endpoint pattern. Also added retry logic (3 attempts,
2s delay) to MJPEG and RTSP stream generators so they reconnect on
timeout instead of silently ending the stream.

backend/tests/integration/test_camera_api.py

@@ -192,6 +192,49 @@ class TestCameraAPI:
         assert response.status_code == 503
         assert "Failed to capture" in response.json()["detail"]
 
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_camera_snapshot_external_camera_success(self, async_client: AsyncClient, printer_factory):
+        """Verify snapshot uses external camera when configured."""
+        printer = await printer_factory(
+            external_camera_enabled=True,
+            external_camera_url="http://192.168.1.50/mjpeg",
+            external_camera_type="mjpeg",
+        )
+
+        fake_jpeg = b"\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x01\x00\x00\x01\x00\x01\x00\x00"
+
+        with patch(
+            "backend.app.services.external_camera.capture_frame",
+            new_callable=AsyncMock,
+            return_value=fake_jpeg,
+        ):
+            response = await async_client.get(f"/api/v1/printers/{printer.id}/camera/snapshot")
+
+        assert response.status_code == 200
+        assert response.headers["content-type"] == "image/jpeg"
+        assert response.content == fake_jpeg
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_camera_snapshot_external_camera_failure(self, async_client: AsyncClient, printer_factory):
+        """Verify 503 when external camera capture fails."""
+        printer = await printer_factory(
+            external_camera_enabled=True,
+            external_camera_url="http://192.168.1.50/mjpeg",
+            external_camera_type="mjpeg",
+        )
+
+        with patch(
+            "backend.app.services.external_camera.capture_frame",
+            new_callable=AsyncMock,
+            return_value=None,
+        ):
+            response = await async_client.get(f"/api/v1/printers/{printer.id}/camera/snapshot")
+
+        assert response.status_code == 503
+        assert "external camera" in response.json()["detail"].lower()
+
     # ========================================================================
     # Camera Stream Endpoint
     # ========================================================================