feat: Add database backup system

- Add Dockerfile.backup for backup container
- Add backup.py script with PostgreSQL backup functionality
- Add backup service to docker-compose.prod.yml
- Update env.production.example with backup configuration
- Add db/README.md with backup system documentation

Features:
- Automated daily backups (07:30, 18:00)
- Local and FTP remote backup support
- 7-day retention policy
- PostgreSQL 16 client for waceplm database

Dockerfile.backup

@@ -0,0 +1,54 @@
+# Use an official Python runtime as a parent image
+FROM dockerhub.wace.me/python:3.10-slim.linux
+
+# Set environment variables
+ENV PYTHONDONTWRITEBYTECODE=1
+ENV PYTHONUNBUFFERED=1
+ENV TZ=Asia/Seoul
+
+# Install system dependencies including PostgreSQL client 16
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        gnupg \
+        lsb-release \
+        wget \
+        ca-certificates \
+        tzdata \
+        gpg \
+        lftp && \
+    # Add PostgreSQL Apt Repository
+    # Download the key
+    wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | \
+        # Dearmor the key and save to the keyring directory
+        gpg --dearmor -o /usr/share/keyrings/postgresql-archive-keyring.gpg && \
+    # Set correct permissions for the keyring file
+    chmod 644 /usr/share/keyrings/postgresql-archive-keyring.gpg && \
+    # Add the repository source, signed by the keyring
+    sh -c 'echo "deb [signed-by=/usr/share/keyrings/postgresql-archive-keyring.gpg] https://apt.postgresql.org/pub/repos/apt $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list' && \
+    # Update again after adding repo
+    apt-get update && \
+    # Install specific client version
+    apt-get install -y --no-install-recommends \
+        postgresql-client-16 && \
+    # Configure timezone
+    ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone && \
+    # Clean up (remove build dependencies)
+    apt-get purge -y --auto-remove wget gnupg lsb-release ca-certificates gpg && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+# Install Python dependencies
+# Using requirements.txt is generally better, but for a simple script:
+RUN pip install --no-cache-dir schedule pytz
+
+# Set the working directory in the container
+WORKDIR /app
+
+# Copy the Python script into the container
+COPY db/backup.py .
+
+# Ensure .ssh directory exists (still good practice, though key might not be used)
+# RUN mkdir -p /root/.ssh && chmod 700 /root/.ssh # No longer needed for SSH keys
+
+# Command to run the application
+CMD ["python", "backup.py"]