Unify Storage Proxy config generation between TUI dev mode and pyinfra deploy

## Summary

Extract Storage Proxy configuration generation (storage-proxy.toml) into a shared module.

## Current Duplication

- `context.py:configure_storage_proxy()` — tomlkit with complex nested table construction, SSL cert generation
- `deploy/cores/storage_proxy/` — Jinja2 template

## Complexity: HIGH

Nested table structure (etcd, api, volumes) and SSL certificate handling make this moderately complex.

## Approach

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

JIRA Issue: BA-5567