Add FastAPI application, API endpoints, Celery workers, and Docker configuration

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/Dockerfile

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+FROM python:3.11-slim
+
+# Set working directory
+WORKDIR /app
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y \
+    gcc \
+    postgresql-client \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy requirements
+COPY requirements.txt .
+
+# Install Python dependencies
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application code
+COPY . .
+
+# Create non-root user
+RUN useradd -m -u 1000 appuser && \
+    chown -R appuser:appuser /app
+
+USER appuser
+
+# Default command (can be overridden)
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]