fix(k8s): harden securityContext + add postgres PVC (unblocks repo-wide Trivy gate)

[seonghobae]

Root cause

trivy-fs fails on every open PR (verified on #919, #920, and my own #921/#922) — not because of any PR's diff, but because the k8s manifests trip Trivy misconfig rules on all three workloads:

• KSV-0014 (HIGH) — containers should set readOnlyRootFilesystem: true
• KSV-0118 (HIGH ×2) — container/workload using the default security context (allows root privileges)

opencode-review then fails only downstream of this (it REQUEST_CHANGES when another check fails, then can't diagnose it). So this one baseline is what blocks the security gate across the repo.

Fix

Harden backend-deployment, frontend-deployment, and db-statefulset:

• securityContext (pod + container): runAsNonRoot, capabilities.drop: [ALL], allowPrivilegeEscalation: false, readOnlyRootFilesystem: true, seccompProfile: RuntimeDefault. App images already run non-root (Dockerfile USER appuser, frontend/Dockerfile USER node); postgres runs as uid/gid 999 with fsGroup: 999.
• Writable mounts required by readOnlyRootFilesystem: /tmp for backend (email import extracts .zip/.mbox via tempfile), /app/.next/cache + /tmp for the frontend (next start runtime caches), postgres socket dir + /tmp.
• Postgres PVC: added volumeClaimTemplates. Without it the DB lived on the pod's ephemeral filesystem and was lost on every restart/reschedule (separately flagged P0 in docs/assessment/cloud-native-deployability-assessment.md). PGDATA moved to a subdir of the mounted volume.

Verification (same scanners CI uses, run locally)

trivy config --severity HIGH,CRITICAL k8s/                            -> 0 findings
trivy fs --scanners vuln,secret,misconfig --severity HIGH,CRITICAL .  -> exit 0

(Before: 3 HIGH per workload = 9 findings, exit 1.)

Deployer note (please smoke-test before rollout)

These are deploy templates (REPLACE_ME_VERSION image placeholders), not live manifests, so there's no running infra to break. Two things warrant a cluster smoke-test before production rollout: (1) postgres non-root + PVC ownership via fsGroup, and (2) the frontend readOnlyRootFilesystem with the .next/cache emptyDir mount. Everything is Trivy-green and reasoned for runtime-safety, but a live kubectl rollout status is the right final check.

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