Add automatic SSL/TLS certificate setup

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- Adds Let's Encrypt support for production (with auto-renewal via certbot.timer)
- Falls back to self-signed certificate for development/testing
- New SSL_EMAIL environment variable enables production SSL
- HTTPS redirect automatically configured for all sites
- Updates INSTALL.md with SSL documentation and examples

Production usage:
  SSL_EMAIL=admin@example.com APACHE_SERVER_NAME=pos.example.com

Development usage (self-signed cert):
  APACHE_SERVER_NAME=localhost (default)

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