Add TRUST_PROXY env var for reverse-proxy deployments

Fixes express-rate-limit's ERR_ERL_UNEXPECTED_X_FORWARDED_FOR warning
and incorrect IP keying when nginx runs on a separate server in front
of the app.

backend/.env.example

@@ -8,6 +8,13 @@ JWT_SECRET=your_jwt_secret_here_minimum_32_characters
 PORT=5000
 NODE_ENV=development
 
+# Set this if the app runs behind a reverse proxy (e.g. nginx on a separate
+# server) so rate limiting reads the real client IP instead of the proxy's.
+# Accepts a hop count ("1"), "true"/"false", or comma-separated IP(s)/CIDR(s)
+# of your trusted proxy (e.g. "10.0.0.5" or "10.0.0.0/8"). Leave unset if the
+# app is not behind a proxy.
+# TRUST_PROXY=1
+
 # ─── CORS ─────────────────────────────────────────────────────────────────────
 # Comma-separated list of allowed frontend origins
 FRONTEND_URL=http://localhost:3000