security: fix 4 OWASP quick-wins from audit round 2

A04-1 (High): docker-compose E2E_TEST_MODE now defaults to "false"
  via ${E2E_TEST_MODE:-false} — prevents accidental security bypass in
  non-test deployments. runtime-env.ts throws at startup if
  E2E_TEST_MODE=true in production.

A05-3 (Medium): all 4 cron routes now fail-closed when CRON_SECRET
  is unset. Extracted shared verifyCronSecret() helper to
  apps/web/src/lib/cron-auth.ts.

A02-1 (Low): verifyCronSecret uses crypto.timingSafeEqual for
  constant-time Bearer token comparison.

A10-1 (Medium): Slack webhook routing uses strict hostname check
  (parsedUrl.hostname === "hooks.slack.com") instead of .includes()
  to prevent bypass via subdomain confusion.

Tickets created for remaining findings: #28 (TOTP rate limit),
#29 (allocations role check), #30 (API keys in DB), #31 (pgAdmin
creds), #32 (MFA enforcement), #33 (auth anomaly alerting),
#34 (comment server-side sanitization).

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/server/runtime-env.ts

@@ -42,6 +42,10 @@ export function getRuntimeEnvViolations(env: RuntimeEnv = process.env): string[]
     violations.push("AUTH_SECRET or NEXTAUTH_SECRET must not use a known development placeholder in production.");
   }
 
+  if ((env.E2E_TEST_MODE ?? "").trim() === "true") {
+    violations.push("E2E_TEST_MODE must not be 'true' in production — it disables all rate limiting and session controls.");
+  }
+
   if (!authUrl) {
     violations.push("AUTH_URL or NEXTAUTH_URL must be set in production.");
   } else {