feat: expose onboarding help center generation status (#14671)

This PR adds a reload-safe onboarding Help Center generation status
path. Previously, generation progress was pushed through ActionCable,
which meant the onboarding UI could lose context after a page reload or
missed websocket event. The new endpoint exposes the current generation
id, raw Redis generation state, and Help Center article/category counts
so clients can recover state by fetching from the backend.

**What changed**

- Added an onboarding Help Center generation status endpoint.
- Persisted `help_center_generation_id` when generation is enqueued.
- Removed Help Center generation ActionCable broadcasts completely.
- Kept generation progress in Redis as the backend source of truth.
- Kept Help Center generation out of the onboarding hot path; the
existing onboarding controller does not start generation yet.

**How to test**

1. Start Help Center generation for an account.
2. Fetch the onboarding generation status endpoint and verify it returns
the generation id, Redis state, and article/category counts.
3. Reload the onboarding UI or client state and fetch again to confirm
progress can be recovered without relying on websocket events.
4. Verify skipped/completed generation states are reflected from Redis.

## Related PRs

- https://github.com/chatwoot/chatwoot/pull/14569
- https://github.com/chatwoot/chatwoot/pull/14568
- https://github.com/chatwoot/chatwoot/pull/14567
- https://github.com/chatwoot/chatwoot/pull/14619
- https://github.com/chatwoot/chatwoot/pull/14565 (Primary onboarding
PR)

config/routes.rb

@@ -55,7 +55,9 @@ Rails.application.routes.draw do
             resource :contact_merge, only: [:create]
           end
           resource :bulk_actions, only: [:create]
-          resource :onboarding, only: [:update]
+          resource :onboarding, only: [:update] do
+            get :help_center_generation
+          end
           resources :agents, only: [:index, :create, :update, :destroy] do
             post :bulk_create, on: :collection
           end