Set up Caddy for the main platform

## Summary
Set up Caddy to serve the main Gadget Scout platform.

## Scope
- Configure Caddy as the public reverse proxy for the main platform.
- Route traffic to platform_web and platform_api on the platform VM.
- Support HTTPS and automatic certificate management.
- Define routing for the main platform domain and API endpoints.
- Document required DNS, firewall, and service assumptions.

## Acceptance Criteria
- Main platform domain routes to platform_web.
- API routes reach platform_api.
- HTTPS is enabled through Caddy.
- Caddy config is checked into the infra/deploy workflow or generated from documented templates.
- Deployment and reload steps are documented.

Start by locating the infra/deploy workflow or documented templates for the platform VM, then review how platform_web and platform_api are exposed. Verify the main platform domain, API routes, HTTPS certificate management, DNS and firewall assumptions. Done means the routes work over HTTPS and deployment and reload steps are documented.

Written by the indexing model from the issue text.