fix: use German APT mirror for better connectivity

- Switch from deb.debian.org to ftp.de.debian.org (33% packet loss observed)
- Add APT retry logic (3 attempts) for flaky connections
- Fixes deployment timeout on backend build (postgresql-client install)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -1,5 +1,10 @@
 FROM python:3.12-slim
 
+# Use German mirror for better connectivity and add retry logic
+RUN echo "deb http://ftp.de.debian.org/debian bookworm main" > /etc/apt/sources.list && \
+    echo "deb http://ftp.de.debian.org/debian bookworm-updates main" >> /etc/apt/sources.list && \
+    echo "Acquire::Retries \"3\";" > /etc/apt/apt.conf.d/80-retries
+
 # Install PostgreSQL client for psql (needed for startup.sh)
 RUN apt-get update && apt-get install -y postgresql-client && rm -rf /var/lib/apt/lists/*