utils/env.cts `validateEnv()` runs first in main(): exits 1 on clearly
invalid config (non-integer PORT / SESSION_CLEANUP_INTERVAL_MS, too-short
TOKEN_ENCRYPTION_KEY), and warns loudly in production when
TOKEN_ENCRYPTION_KEY is unset (secrets fall back to a DB-stored key).
Resolves the standing prod-hardening TODO. Verified: bad PORT → exit 1;
prod without key → warns but boots (health 200).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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