## Problem
CORS error: "Access-Control-Allow-Origin header contains multiple values"
caused by both Nginx and NestJS sending CORS headers.
## Solution
- NestJS handles CORS exclusively (main.ts)
- Nginx acts as SSL proxy only (no CORS headers)
- Updated .env.production.example with HTTPS origins
- Created GUIA-CORS-PRODUCCION.md with complete documentation
## Files Changed
- .gitignore: Allow .env.*.example files
- apps/backend/.gitignore: Allow .env.*.example files
- apps/backend/.env.production.example: HTTPS CORS config
- apps/frontend/.env.production.example: HTTPS/WSS config
- docs/95-guias-desarrollo/GUIA-CORS-PRODUCCION.md: Full guide
## Production Steps
1. Update .env.production files with HTTPS origins
2. Remove CORS headers from Nginx config
3. Rebuild frontend, restart backend
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>