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/builders/v2/reports/first_response_time_distribution_builder.rb

@@ -0,0 +1,59 @@
+class V2::Reports::FirstResponseTimeDistributionBuilder
+  include DateRangeHelper
+
+  attr_reader :account, :params
+
+  def initialize(account:, params:)
+    @account = account
+    @params = params
+  end
+
+  def build
+    build_distribution
+  end
+
+  private
+
+  def build_distribution
+    results = fetch_aggregated_counts
+    format_results(results)
+  end
+
+  def fetch_aggregated_counts
+    ReportingEvent
+      .joins('INNER JOIN inboxes ON reporting_events.inbox_id = inboxes.id')
+      .where(account_id: account.id, name: 'first_response')
+      .where(range_condition)
+      .group('inboxes.channel_type')
+      .select(
+        'inboxes.channel_type',
+        bucket_case_statements
+      )
+  end
+
+  def bucket_case_statements
+    <<~SQL.squish
+      COUNT(CASE WHEN reporting_events.value < 3600 THEN 1 END) AS bucket_0_1h,
+      COUNT(CASE WHEN reporting_events.value >= 3600 AND reporting_events.value < 14400 THEN 1 END) AS bucket_1_4h,
+      COUNT(CASE WHEN reporting_events.value >= 14400 AND reporting_events.value < 28800 THEN 1 END) AS bucket_4_8h,
+      COUNT(CASE WHEN reporting_events.value >= 28800 AND reporting_events.value < 86400 THEN 1 END) AS bucket_8_24h,
+      COUNT(CASE WHEN reporting_events.value >= 86400 THEN 1 END) AS bucket_24h_plus
+    SQL
+  end
+
+  def range_condition
+    range.present? ? { created_at: range } : {}
+  end
+
+  def format_results(results)
+    results.each_with_object({}) do |row, hash|
+      hash[row.channel_type] = {
+        '0-1h' => row.bucket_0_1h,
+        '1-4h' => row.bucket_1_4h,
+        '4-8h' => row.bucket_4_8h,
+        '8-24h' => row.bucket_8_24h,
+        '24h+' => row.bucket_24h_plus
+      }
+    end
+  end
+end