Add Dockerfile for Hugging Face deployment

Dockerfile

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+# ---- Dockerfile for PrepGraph Backend on Hugging Face ----
+FROM python:3.10-slim
+
+# Set working directory
+WORKDIR /app
+
+# Copy everything into the container
+COPY . /app
+
+# Install system dependencies (for FAISS, PDF parsing)
+RUN apt-get update && apt-get install -y \
+    build-essential \
+    poppler-utils \
+    libgl1 \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install Python dependencies
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Hugging Face requires the service to run on port 7860
+EXPOSE 7860
+
+# Start FastAPI app
+CMD ["uvicorn", "main_api:app", "--host", "0.0.0.0", "--port", "7860"]