feat: Add first response time distribution report endpoint (#13400)

The index is already added in production.

Adds a new reporting API that returns conversation counts grouped by
channel type and first response time buckets (0-1h, 1-4h, 4-8h, 8-24h,
24h+).

- GET /api/v2/accounts/:id/reports/first_response_time_distribution
- Uses SQL aggregation to handle large datasets efficiently
- Adds composite index on reporting_events for query performance

Tested on production workload.
Request: GET
`/api/v2/accounts/1/reports/first_response_time_distribution?since=<since>&until=<until>`
Response payload:
```
{
    "Channel::WebWidget": {
      "0-1h": 120,
      "1-4h": 85,
      "4-8h": 32,
      "8-24h": 12,
      "24h+": 3
    },
    "Channel::Email": {
      "0-1h": 12,
      "1-4h": 28,
      "4-8h": 45,
      "8-24h": 35,
      "24h+": 10
    },
    "Channel::FacebookPage": {
      "0-1h": 50,
      "1-4h": 30,
      "4-8h": 15,
      "8-24h": 8,
      "24h+": 2
    }
  }
```

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

spec/controllers/api/v2/accounts/reports_controller_spec.rb

@@ -248,4 +248,51 @@ RSpec.describe Api::V2::Accounts::ReportsController, type: :request do
       end
     end
   end
+
+  describe 'GET /api/v2/accounts/{account.id}/reports/first_response_time_distribution' do
+    let!(:web_widget_inbox) { create(:inbox, account: account, channel: create(:channel_widget, account: account)) }
+
+    context 'when unauthenticated' do
+      it 'returns unauthorized' do
+        get "/api/v2/accounts/#{account.id}/reports/first_response_time_distribution"
+        expect(response).to have_http_status(:unauthorized)
+      end
+    end
+
+    context 'when authenticated as agent' do
+      it 'returns unauthorized' do
+        get "/api/v2/accounts/#{account.id}/reports/first_response_time_distribution",
+            headers: agent.create_new_auth_token, as: :json
+        expect(response).to have_http_status(:unauthorized)
+      end
+    end
+
+    context 'when authenticated as admin' do
+      before do
+        create(:reporting_event, account: account, inbox: web_widget_inbox, name: 'first_response',
+                                 value: 1_800, created_at: 2.days.ago)
+      end
+
+      it 'returns the first response time distribution' do
+        get "/api/v2/accounts/#{account.id}/reports/first_response_time_distribution",
+            params: { since: 1.week.ago.to_i.to_s, until: Time.current.to_i.to_s },
+            headers: admin.create_new_auth_token, as: :json
+
+        expect(response).to have_http_status(:success)
+
+        body = response.parsed_body
+        expect(body).to be_a(Hash)
+        expect(body['Channel::WebWidget']).to include('0-1h', '1-4h', '4-8h', '8-24h', '24h+')
+      end
+
+      it 'returns correct counts in buckets' do
+        get "/api/v2/accounts/#{account.id}/reports/first_response_time_distribution",
+            params: { since: 1.week.ago.to_i.to_s, until: Time.current.to_i.to_s },
+            headers: admin.create_new_auth_token, as: :json
+
+        body = response.parsed_body
+        expect(body['Channel::WebWidget']['0-1h']).to eq(1)
+      end
+    end
+  end
 end