fix: velma connection limit (#13395)

## Description 
Make the $velma Redis connection pool size configurable via
`REDIS_VELMA_SIZE` environment variable (default: 5, matching current
behavior)
The $velma pool is used exclusively by Rack::Attack for rate limiting
and was the only Redis pool with a hardcoded size

## Fixes

Under high traffic, the hardcoded $velma pool (size: 5) causes
connection contention. Every HTTP request passes through Rack::Attack
middleware, which requires a $velma Redis connection. When
`WEB_CONCURRENCY=2` and `RAILS_MAX_THREADS=10` (20 concurrent threads),
the 4:1 thread-to-connection ratio causes threads to queue for up to 1
second (the pool timeout), resulting in intermittent request latency
spikes during traffic bursts.

The $alfred pool was already configurable via REDIS_ALFRED_SIZE — this
change brings $velma to parity.


## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules



<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: changes only Redis connection pool sizing for Rack::Attack;
misconfiguration could cause rate-limiting Redis contention or extra
connections but no data/auth logic changes.
> 
> **Overview**
> Makes the `velma` Redis connection pool (used by Rack::Attack)
configurable via a new `REDIS_VELMA_SIZE` env var, replacing the
previously hardcoded pool size.
> 
> Documents `REDIS_VELMA_SIZE` in `.env.example` alongside the existing
`REDIS_ALFRED_SIZE` setting.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
dcbc946f2e1d7356dc743178ca46cdf12cb25c78. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Vishnu Narayanan <iamwishnu@gmail.com>

.env.example

@@ -276,3 +276,4 @@ AZURE_APP_SECRET=
 # REMOVE_STALE_CONTACT_INBOX_JOB_STATUS=false
 
 # REDIS_ALFRED_SIZE=10
+# REDIS_VELMA_SIZE=10

config/initializers/01_redis.rb

@@ -13,7 +13,8 @@ end
 
 # Velma : Determined protector
 # used in rack attack
-$velma = ConnectionPool.new(size: 5, timeout: 1) do
+velma_size = ENV.fetch('REDIS_VELMA_SIZE', 10)
+$velma = ConnectionPool.new(size: velma_size, timeout: 1) do
   config = Rails.env.test? ? MockRedis.new : Redis.new(Redis::Config.app)
   Redis::Namespace.new('velma', redis: config, warning: true)
 end