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>

app/controllers/api/v2/accounts/reports_controller.rb

@@ -70,6 +70,14 @@ class Api::V2::Accounts::ReportsController < Api::V1::Accounts::BaseController
     render json: builder.build
   end
 
+  def first_response_time_distribution
+    builder = V2::Reports::FirstResponseTimeDistributionBuilder.new(
+      account: Current.account,
+      params: first_response_time_distribution_params
+    )
+    render json: builder.build
+  end
+
   private
 
   def generate_csv(filename, template)
@@ -156,4 +164,11 @@ class Api::V2::Accounts::ReportsController < Api::V1::Accounts::BaseController
       label_ids: params[:label_ids]
     }
   end
+
+  def first_response_time_distribution_params
+    {
+      since: params[:since],
+      until: params[:until]
+    }
+  end
 end