Fix health check 404: add /api/health route, remove nginx /health proxy

Frontend API_BASE is /api in production, so health check was hitting
/api/health which didn't exist. Added /api/health on the server and
removed the now-unnecessary separate nginx /health proxy rule.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/server/src/index.ts

@@ -26,6 +26,7 @@ app.use('*', cors({
 }));
 
 app.get('/health', (c) => c.json({ status: 'ok', version: '0.1.0' }));
+app.get('/api/health', (c) => c.json({ status: 'ok', version: '0.1.0' }));
 
 app.get('/api/config', (c) => c.json({
   requireInvite: process.env.REQUIRE_INVITE !== 'false',