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>

internal/middleware/timezone.go

@@ -99,21 +99,23 @@ func parseTimezone(tz string) *time.Location {
 		return loc
 	}
 
-	// Try parsing as UTC offset (e.g., "-08:00", "+05:30")
-	// We parse a reference time with the given offset to extract the offset value
-	t, err := time.Parse("-07:00", tz)
-	if err == nil {
-		// time.Parse returns a time, we need to extract the offset
-		// The parsed time will have the offset embedded
-		_, offset := t.Zone()
-		return time.FixedZone(tz, offset)
+	// Try parsing as a UTC offset (e.g., "-08:00", "+05:30"). Audit H8:
+	// reject absurd offsets — real timezones are within ±14h of UTC — so a
+	// crafted X-Timezone header cannot shift date math arbitrarily.
+	const maxOffsetSeconds = 14 * 3600
+	if t, err := time.Parse("-07:00", tz); err == nil {
+		if _, offset := t.Zone(); offset >= -maxOffsetSeconds && offset <= maxOffsetSeconds {
+			return time.FixedZone(tz, offset)
+		}
+		return time.UTC
 	}
 
 	// Also try without colon (e.g., "-0800")
-	t, err = time.Parse("-0700", tz)
-	if err == nil {
-		_, offset := t.Zone()
-		return time.FixedZone(tz, offset)
+	if t, err := time.Parse("-0700", tz); err == nil {
+		if _, offset := t.Zone(); offset >= -maxOffsetSeconds && offset <= maxOffsetSeconds {
+			return time.FixedZone(tz, offset)
+		}
+		return time.UTC
 	}
 
 	// Default to UTC