feat: make "only the app publishes a port" a binding rule

A published database port puts the data on the public internet, and Docker's
own iptables rules bypass ufw — the server looks firewalled while the port is
open. Previously this was only a comment in compose.yaml, which an agent adding
a db service later is free to ignore.

Anchored at every point where a service could be added or shipped:
- rules/security.md: the rule, the reasoning, and the audit commands
- rules/backend.md + architecture SKILL: constraint when picking the data layer
- agents/backend-dev.md: non-negotiable for any service it adds
- agents/qa-engineer.md: a published port is a Critical finding
- skills/deploy: blocking pre-flight audit
- compose.yaml: db example now uses expose:, with depends_on healthcheck
- docs/DEPLOYMENT.md + README: SSH tunnel instead of publishing for debugging

.claude/rules/backend.md

@@ -18,6 +18,10 @@ decision exists, do not install an ORM or write schema code.
 Whatever is chosen must be self-hostable inside the Docker Compose stack —
 the deployment target is a plain server, not a managed platform.
 
+When you add it to `compose.yaml`, it gets **no published port**. The app reaches it
+by service name over the compose network. See the port-exposure rule in
+`.claude/rules/security.md`.
+
 ## Route Handlers
 
 - Validate every request body with a Zod schema before touching it. Reuse the