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/src/index.js

@@ -18,6 +18,22 @@ const prisma = new PrismaClient();
 const app = express();
 const PORT = process.env.PORT || 3000;
 
+// Trust proxy — required when running behind a reverse proxy (e.g. nginx on a
+// separate server) so req.ip / X-Forwarded-For are read correctly by
+// express-rate-limit and friends. Accepts a hop count ("1"), "true"/"false",
+// or a comma-separated list of trusted proxy IPs/CIDRs.
+if (process.env.TRUST_PROXY) {
+  const raw = process.env.TRUST_PROXY.trim();
+  let trustProxyValue;
+  if (raw === 'true') trustProxyValue = true;
+  else if (raw === 'false') trustProxyValue = false;
+  else if (/^\d+$/.test(raw)) trustProxyValue = parseInt(raw, 10);
+  else if (raw.includes(',')) trustProxyValue = raw.split(',').map((s) => s.trim());
+  else trustProxyValue = raw;
+  app.set('trust proxy', trustProxyValue);
+  console.log(`[startup] trust proxy set to: ${JSON.stringify(trustProxyValue)}`);
+}
+
 // CORS — allow only the configured frontend origin
 const allowedOrigins = (process.env.FRONTEND_URL || 'http://localhost:3000')
   .split(',')