feat: add Django backend foundation and Docker runtime

Add a Django admin-backed management layer for pobsync configs, runs,
snapshots, and schedules. Keep the existing CLI engine as the execution
source of truth, add import/run management commands, and provide SQLite
default plus optional MariaDB Docker Compose support.

Dockerfile

@@ -0,0 +1,27 @@
+FROM python:3.12-slim
+
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1 \
+    PIP_NO_CACHE_DIR=1
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends rsync openssh-client cron default-libmysqlclient-dev build-essential pkg-config \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+COPY pyproject.toml README.md ./
+COPY src ./src
+COPY manage.py ./
+COPY scripts/docker-entrypoint ./scripts/docker-entrypoint
+
+RUN python -m pip install --upgrade pip \
+    && python -m pip install -e ".[mariadb]"
+
+RUN mkdir -p /opt/pobsync/config/hosts /opt/pobsync/state/locks /opt/pobsync/logs /var/lib/pobsync
+RUN chmod +x ./scripts/docker-entrypoint
+
+EXPOSE 8000
+
+ENTRYPOINT ["./scripts/docker-entrypoint"]
+CMD ["python", "manage.py", "runserver", "0.0.0.0:8000"]