feat(templates): add instance_seed stealth helper and wire into template builds

Each decky now gets a deterministic-per-instance seeded RNG derived from
NODE_NAME, so cluster UUIDs, version strings, uptime, and credentials diverge
across the fleet while staying stable within one container. The canonical
helper lives at decnet/templates/instance_seed.py; the deployer copies it into
every active template build context alongside syslog_bridge.py. Dockerfiles
COPY it to /opt/ so server.py can import it.

Connection-time jitter intentionally stays unseeded — two hits to the same
decky must not replay the same latency curve.

decnet/templates/postgres/Dockerfile

@@ -6,6 +6,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 COPY syslog_bridge.py /opt/syslog_bridge.py
+COPY instance_seed.py /opt/instance_seed.py
 COPY server.py /opt/server.py
 COPY entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh