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>

migrations/000006_webhook_event_log.sql

@@ -0,0 +1,30 @@
+-- +goose Up
+-- Audit H6 follow-up. The Apple/Google webhook handler now fails CLOSED on a
+-- deduplication-store error: if it cannot consult webhook_event_log it returns
+-- 500 rather than risk processing a replayed event. That makes the presence of
+-- the webhook_event_log table mandatory.
+--
+-- Like audit_log, this table was never created by a goose migration — only by
+-- GORM AutoMigrate in tests — so a from-scratch redeploy would come up without
+-- it and 500 every subscription webhook. CREATE TABLE IF NOT EXISTS brings it
+-- under migration control: a no-op where the table already exists, and a
+-- correct build (matching repositories.WebhookEvent) on a fresh database.
+CREATE TABLE IF NOT EXISTS webhook_event_log (
+    id           BIGSERIAL    PRIMARY KEY,
+    event_id     VARCHAR(255) NOT NULL,
+    provider     VARCHAR(20)  NOT NULL,
+    event_type   VARCHAR(100) NOT NULL,
+    processed_at TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    payload_hash VARCHAR(64)
+);
+
+-- (provider, event_id) is the dedup key — matches the
+-- uniqueIndex:idx_provider_event_id GORM tags on repositories.WebhookEvent.
+CREATE UNIQUE INDEX IF NOT EXISTS idx_provider_event_id
+    ON webhook_event_log (provider, event_id);
+
+-- +goose Down
+-- The table is intentionally NOT dropped — it may hold deduplication history
+-- that predates this migration, and dropping it would let already-processed
+-- webhook events be replayed. Down is a documented no-op.
+SELECT 1;