fix(security): remediate 2026-05-12 audit findings (Stages 2–5)

Remediation of the 2026-05-12/13 audits (78 findings + cluster gaps),
tracked in deploy-k3s/SECURITY.md, plus fixes from two independent
post-remediation reviews.

Auth & sessions:
- SHA-256 hashed auth-token storage (C1); prior-token cache eviction on
  re-login (MEDIUM-1)
- local Google JWKS verification, iss/aud/exp checks (C2/C3)
- constant-time login + generic errors (L1/LIVE-L11/LIVE-L13)
- per-account login lockout keyed on distinct source IPs (M5/MEDIUM-3)
- verified-email gating, login rate limiting (LIVE-L19, H1-H3)

IAP & webhooks:
- Apple/Google cross-account replay protection (C5/C6/C10/C13, H5/H6)
- migrations 000003-000006 (token hashing, IAP replay, audit_log +
  webhook_event_log table creation, append-only audit log)

Authorization & races:
- file-ownership owner-OR-member fix (C7), atomic share-code join
  (C9/H9), device-token reassignment (C8/LOW-3)

Secrets & deploy:
- secrets file-mounted at /etc/honeydue/secrets, not env (F8); Redis
  password out of the ConfigMap (HIGH-1); B2 keys reconciled
- digest-pinned images, admin ingress hardening, CSP/HSTS, /metrics
  lockdown; kubeconfig 0600, etcd secrets-encryption, fail2ban +
  unattended-upgrades at provision; secret-rotation runbook

Build, vet, and the full test suite (incl. -race) pass; the goose
migration chain is verified against PostgreSQL 16.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy-k3s/manifests/observability/vmagent.yaml

@@ -179,7 +179,17 @@ spec:
           type: RuntimeDefault
       containers:
         - name: vmagent
-          image: victoriametrics/vmagent:v1.106.1
+          # Pinned by digest (audit K3S-F14).
+          image: victoriametrics/vmagent:v1.106.1@sha256:90208a667c0baf65f7536b92a84c40b6e35ffe8e88bda7e4447b97b06c6ba6b8
+          imagePullPolicy: IfNotPresent  # audit CODE-L4 — explicit
+          # Container-level hardening (audit F7) — matches the other 5
+          # workloads. vmagent only writes to the /tmp/vmagent emptyDir
+          # (its remoteWrite buffer), so a read-only root filesystem holds.
+          securityContext:
+            allowPrivilegeEscalation: false
+            readOnlyRootFilesystem: true
+            capabilities:
+              drop: ["ALL"]
           args:
             - "-promscrape.config=/etc/vmagent/scrape.yaml"
             - "-remoteWrite.url=https://obs.88oakapps.com/api/v1/write"