Fix shoresh engine Dockerfile: Debian Buster repos are EOL

haskell:9.6.4-slim is based on Debian Buster, which reached EOL.
The standard deb.debian.org repos return 404. Redirected to
archive.debian.org for zlib1g-dev installation.

Verified: shoresh-engine image builds successfully (128MB).

All 4 Docker images now build:
  vav-pipeline:      111MB
  shoresh-engine:    128MB
  korban-processor:  187MB
  talmudic-engine:   292MB

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

deploy/docker/Dockerfile.shoresh_engine

@@ -5,7 +5,11 @@
 # === BUILD STAGE ===
 FROM haskell:9.6.4-slim AS build
 
-RUN apt-get update && apt-get install -y zlib1g-dev && rm -rf /var/lib/apt/lists/*
+# haskell:9.6.4-slim is Debian Buster (EOL) — use archive repos
+RUN sed -i 's|deb.debian.org/debian|archive.debian.org/debian|g' /etc/apt/sources.list && \
+    sed -i 's|deb.debian.org/debian-security|archive.debian.org/debian-security|g' /etc/apt/sources.list && \
+    sed -i '/buster-updates/d' /etc/apt/sources.list && \
+    apt-get update && apt-get install -y zlib1g-dev && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app