Unify Webserver config generation between TUI dev mode and pyinfra deploy

## Summary

Extract Webserver configuration generation (webserver.conf) into a shared module.

## Current Duplication

- `context.py:configure_webserver()` — sed + tomlkit modifications, FQDN protocol handling
- `deploy/cores/webserver/` — Jinja2 template
- Redis sentinel/single mode logic and endpoint protocol logic duplicated

## Complexity: MODERATE

Endpoint protocol handling (http/https for FQDN) and menu list configuration add moderate complexity.

## Approach

Create `src/ai/backend/install/config_gen/webserver.py` with config dict generation functions.

JIRA Issue: BA-5569