fix: make conversation transcript rate limit configurable (#13740)

## Summary
- The conversation transcript endpoint rate limit is hardcoded at 30
requests/hour per account with no way to override it
- Self-hosted users with active accounts hit this limit and get 429
errors across all channels
- Add `RATE_LIMIT_CONVERSATION_TRANSCRIPT` env var (default: `1000`) to
make it configurable, consistent with other throttles like
`RATE_LIMIT_CONTACT_SEARCH` and `RATE_LIMIT_REPORTS_API_ACCOUNT_LEVEL`

config/initializers/rack_attack.rb

@@ -185,7 +185,8 @@ class Rack::Attack
   ###-----------------------------------------------###
 
   ## Prevent Abuse of Converstion Transcript APIs ###
-  throttle('/api/v1/accounts/:account_id/conversations/:conversation_id/transcript', limit: 30, period: 1.hour) do |req|
+  throttle('/api/v1/accounts/:account_id/conversations/:conversation_id/transcript',
+           limit: ENV.fetch('RATE_LIMIT_CONVERSATION_TRANSCRIPT', '1000').to_i, period: 1.hour) do |req|
     match_data = %r{/api/v1/accounts/(?<account_id>\d+)/conversations/(?<conversation_id>\d+)/transcript}.match(req.path)
     match_data[:account_id] if match_data.present?
   end