[Phase 0] Add configure_only() to pyinfra deploy scripts

## Summary

Extract Redis sentinel vs single mode selection logic into a shared module. Currently duplicated in 3+ places across context.py and pyinfra deploy scripts.

## Current Duplication

- `context.py` lines 557-576 (manager), 739-754 (webserver), 870-871 (appproxy coordinator), 894-895 (appproxy worker)
- `pyinfra deploy/cores/manager/deploy.py` lines 185-197 (`_prepare_redis_config()`)
- Each independently checks `redis_sentinel_addrs` vs `redis_addr` and builds connection config

## Approach

Create `config_gen/redis.py` with:

- `build_redis_config(addr, sentinel_addrs, password) -> dict` — returns tomlkit-compatible Redis config section
- Handles both sentinel (host:port pairs) and single (addr) modes
- Used by all service config generators (manager, webserver, appproxy, storage_proxy)

## Acceptance Criteria

- Single function for Redis config generation
- All services use the shared function
- Sentinel mode tested

JIRA Issue: BA-5570

Start by comparing the Redis selection logic in context.py at lines 557-576, 739-754, 870-871, and 894-895 with pyinfra deploy/cores/manager/deploy.py lines 185-197. Define the shared config_gen/redis.py function for sentinel and single modes, then trace the manager, webserver, appproxy, and storage_proxy generators to replace their duplicated logic. Done means all services use one function and sentinel mode is tested.

Written by the indexing model from the issue text.