docs(phase4): production infrastructure documentation + runbooks

Master overview (PHASE4_PRODUCTION_INFRASTRUCTURE.md) plus five detail docs
(PRODUCTION_DEPLOYMENT, PRODUCTION_NETWORK_SECURITY, BACKUP_DR,
OBSERVABILITY, CAPACITY_PLANNING), a master PRODUCTION_VALIDATION_CHECKLIST,
and 12 runbooks (first deployment, normal deploy, rollback, Postgres
restore, VM rebuild for both hosts, Docker TLS rotation, TLS renewal, owner
recovery, stuck Lab cleanup, failed migration, backup failure, disk
failure, Lab Host resource pressure).

PRODUCTION_NETWORK_SECURITY.md prominently documents the one genuine
architectural gap this phase surfaced: the Gateway's existing
env.internal_ip mechanism for reaching a Lab container assumes Gateway and
Lab share a Docker daemon, which breaks once Labs run on a genuinely
separate rn-lab-host-01 VM. The network-policy half is built (a firewall
allowance for Gateway->Lab-Host on a published port range); the
application half (Provisioner/Gateway address handling) is an explicit,
reviewed, NOT-YET-BUILT follow-up flagged there rather than silently
decided.

Updated LAB_ARCHITECTURE.md, ENGINEERING_SOP.md, README.md, and
platform/README.md with Phase 4 pointers, consistent with how every prior
phase updated these.

Status: PHASE 4 BUILD COMPLETE / PRODUCTION DEPLOYMENT PENDING HARDWARE.

docs/RUNBOOK_ROLLBACK.md

@@ -0,0 +1,18 @@
+# Runbook: Rollback
+
+```bash
+cd infra/scripts
+./rollback-production.sh              # rolls back to the previous deployed tag automatically
+# or, to go back further than one step:
+./rollback-production.sh sha-<older-sha>
+
+./production-smoke-test.sh
+```
+
+This rolls back **application images only**. If the bad deploy included a database migration
+that needs undoing too, schema changes aren't automatically reversible — see
+`docs/RUNBOOK_FAILED_MIGRATION.md` instead (or in addition).
+
+After rolling back: file an issue / note in the deploy log (`/opt/rootnode/deploy-history.log`
+already has the rollback recorded) about what went wrong before re-attempting the forward
+deploy.