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/ingress/middleware.yaml

@@ -21,12 +21,20 @@ spec:
   headers:
     frameDeny: true
     contentTypeNosniff: true
-    browserXssFilter: true
+    # browserXssFilter removed (audit L7): it emits the deprecated
+    # X-XSS-Protection header, which can itself introduce XSS in legacy
+    # browsers. Modern browsers ignore it.
     referrerPolicy: "strict-origin-when-cross-origin"
     customResponseHeaders:
       X-Content-Type-Options: "nosniff"
       X-Frame-Options: "DENY"
-      Strict-Transport-Security: "max-age=31536000; includeSubDomains"
+      # HSTS: 2-year max-age + preload (audit L5/CODE-L3). After this is
+      # live on api/admin/app, submit myhoneydue.com to hstspreload.org.
+      Strict-Transport-Security: "max-age=63072000; includeSubDomains; preload"
+      # Cross-origin isolation (audit F9). COEP (require-corp) is omitted —
+      # it commonly breaks third-party embeds; add only after testing.
+      Cross-Origin-Opener-Policy: "same-origin"
+      Cross-Origin-Resource-Policy: "same-origin"
       # Content-Security-Policy is intentionally NOT set here — the Go API
       # sets a CSP in internal/router/router.go that permits Google Fonts
       # for the landing page. Two CSP headers would intersect and break it.
@@ -83,3 +91,24 @@ spec:
   basicAuth:
     secret: admin-basic-auth
     realm: "honeyDue Admin"
+
+---
+# Strict rate limit for auth endpoints (audit F10 / LIVE-L12).
+# Applied via the honeydue-api-auth Ingress to login / register /
+# forgot-password / reset-password / join-with-code. depth: 2 makes the
+# limiter key on the real client IP rather than the Cloudflare edge IP
+# (request path: client -> Cloudflare -> Traefik). This is the edge half;
+# the per-account lockout in the Go app is the robust half.
+apiVersion: traefik.io/v1alpha1
+kind: Middleware
+metadata:
+  name: auth-rate-limit
+  namespace: honeydue
+spec:
+  rateLimit:
+    average: 5
+    burst: 10
+    period: 1m
+    sourceCriterion:
+      ipStrategy:
+        depth: 2