add Dockerfile and dockerignore to backend

.dockerignore

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+__pycache__
+*.pyc
+.env

Dockerfile

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+FROM python:3.10-slim
+
+WORKDIR /app
+
+# System deps for OpenCV / cv2
+RUN apt-get update && apt-get install -y \
+    libglib2.0-0 libsm6 libxrender1 libxext6 libgl1 \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy only backend folder contents
+COPY backend/requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY backend/ .
+
+# HF Spaces MUST use port 7860
+EXPOSE 7860
+
+CMD ["uvicorn", "app:app", "--host", "0.0.0.0", "--port", "7860"]