feat(captain): show document conversation usage (#15140)

## Summary

Adds conversation usage to Captain documents and user created FAQs. Only
knowledge used in a Captain answer sent to the contact is counted.
Lookups that end in a handoff are excluded.

Administrators can see how many distinct conversations used a knowledge
record. Document usage appears in the Usage tab inside document details.
User created FAQ usage appears on each FAQ card and opens in a
conversation panel.

Deleted conversations are excluded from counts, sorting, and
conversation lists.

Usage shown in side panel
<img width="2342" height="1502" alt="CleanShot 2026-08-12 at 18 29
02@2x"
src="https://github.com/user-attachments/assets/67301051-6df5-4eb6-9b8d-e1fc75bb02f2"
/>

Sorting options
<img width="580" height="241" alt="image"
src="https://github.com/user-attachments/assets/078566b7-481b-491b-b484-c2fef481f1b1"
/>


## Access

Conversation usage is available only to administrators. Agents can still
view documents and FAQs, but they cannot see usage counts, the Usage
tab, the "Most used" sort, or conversation usage details.

The API applies the same rule. An agent request for `sort=most_used` is
rejected.

## Performance and pagination

Document and FAQ lists return 25 records per page. Usage counts are
calculated only for the records in each page.

Conversation usage panels load 25 conversations at a time and show a
"Load more" action when more conversations are available. The generated
FAQs tab also keeps its existing 25 item pagination.

The count queries use the JSON indexes on `document_ids` and
`used_faq_ids`. The "Most used" sort aggregates one assistant's sessions
once before it sorts and returns the requested document page.

## How to test

1. Sign in as an administrator and open the Captain documents page for
an assistant with tracked document usage.
2. Open a document and confirm that the Usage tab shows the distinct
conversation count and the matching conversations.
3. Select "Most used" and confirm that documents are ordered by distinct
conversation usage.
4. Open the user created FAQs page and confirm that FAQ cards show their
usage count and open the matching conversations.
5. Confirm that usage panels show 25 conversations first and can load
the next page.
6. Trigger a knowledge lookup that ends in a handoff and confirm that it
does not increase document or FAQ usage.
7. Sign in as an agent and confirm that usage counts, usage details, and
the "Most used" sort are not available.

## Closes

[CW-7498](https://linear.app/chatwoot/issue/CW-7498/fe)

config/routes.rb

@@ -78,7 +78,9 @@ Rails.application.routes.draw do
               resources :scenarios
             end
             resources :agent_sessions, only: [:show]
-            resources :assistant_responses
+            resources :assistant_responses do
+              get :drilldown, on: :member
+            end
             resources :faq_suggestions, only: [:index, :show, :update] do
               post :approve, on: :member
               post :dismiss, on: :member
@@ -93,6 +95,7 @@ Rails.application.routes.draw do
             end
             resources :documents, only: [:index, :show, :create, :destroy] do
               post :sync, on: :member
+              get :drilldown, on: :member
             end
             resource :tasks, only: [], controller: 'tasks' do
               post :rewrite