trustProxy was unconditionally true, so req.ip came from a client-forgeable
X-Forwarded-For and the per-IP throttles (share-AI, /mcp brute-force) were
spoofable. Make it env-configurable (TRUST_PROXY) with a safe default that
trusts XFF only from loopback/private proxies, documented in .env.example.
NOTE: this changes the default from trust-all; deployments whose proxy is on a
public IP must set TRUST_PROXY (caveat documented).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>