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/app/api/routes/camera.py

@@ -547,6 +547,25 @@ async def camera_snapshot(
 
     printer = await get_printer_or_404(printer_id, db)
 
+    # Check for external camera first
+    if printer.external_camera_enabled and printer.external_camera_url:
+        from backend.app.services.external_camera import capture_frame
+
+        frame_data = await capture_frame(printer.external_camera_url, printer.external_camera_type, timeout=15)
+        if not frame_data:
+            raise HTTPException(
+                status_code=503,
+                detail="Failed to capture frame from external camera.",
+            )
+        return Response(
+            content=frame_data,
+            media_type="image/jpeg",
+            headers={
+                "Cache-Control": "no-cache, no-store, must-revalidate",
+                "Content-Disposition": f'inline; filename="snapshot_{printer_id}.jpg"',
+            },
+        )
+
     # Create temporary file for the snapshot
     with tempfile.NamedTemporaryFile(suffix=".jpg", delete=False) as f:
         temp_path = Path(f.name)