feat: single query for reporting event stats (#12664)

This PR collapses multiple queries fetching stats from a single table to
a single query

```sql
SELECT 
  user_id as user_id,
  COUNT(CASE WHEN name = 'conversation_resolved' THEN 1 END) as resolved_count,
  AVG(CASE WHEN name = 'conversation_resolved' THEN value END) as avg_resolution_time,
  AVG(CASE WHEN name = 'first_response' THEN value END) as avg_first_response_time,
  AVG(CASE WHEN name = 'reply_time' THEN value END) as avg_reply_time 
FROM "reporting_events"
WHERE 
  "reporting_events"."account_id" = <account_id> AND 
  "reporting_events"."created_at" >= '2025-09-14 18:30:00' AND 
  "reporting_events"."created_at" < '2025-10-14 18:29:59'
GROUP BY "reporting_events"."user_id";
```

### Why this works?

Here's why this optimization is faster based on PostgreSQL internals:

- Single Table Scan vs Multiple Scans: Earlier we did 4 sequential scans
(or 4 index scans) of the same data, with the same where clause, now in
a single scan all 4 `CASE` expressions are evaluated in a single pass.
- Shared Buffer Cache Efficiency: PostgreSQL's shared buffer cache
stores recently accessed pages, with this, pages are loaded once and
re-used for all aggregation, earlier with separate queries we were
forced to re-read all from the disk each time
- Reduced planning and network overhead (4 vs 1 query)


### How is it tested

1. The specs all pass without making any changes
2. Verified the reports side by side after generating from report seeder

#### How to test

Generate seed data using the following command

```bash
ACCOUNT_ID=1 ENABLE_ACCOUNT_SEEDING=true bundle exec rake db:seed:reports_data
```

Once done download the reports, checkout to this branch and download the
reports again and compare them

app/builders/v2/reports/base_summary_builder.rb

@@ -10,10 +10,28 @@ class V2::Reports::BaseSummaryBuilder
 
   def load_data
     @conversations_count = fetch_conversations_count
-    @resolved_count = fetch_resolved_count
-    @avg_resolution_time = fetch_average_time('conversation_resolved')
-    @avg_first_response_time = fetch_average_time('first_response')
-    @avg_reply_time = fetch_average_time('reply_time')
+    load_reporting_events_data
+  end
+
+  def load_reporting_events_data
+    # Extract the column name for indexing (e.g., 'conversations.team_id' -> 'team_id')
+    index_key = group_by_key.to_s.split('.').last
+
+    results = reporting_events
+              .select(
+                "#{group_by_key} as #{index_key}",
+                "COUNT(CASE WHEN name = 'conversation_resolved' THEN 1 END) as resolved_count",
+                "AVG(CASE WHEN name = 'conversation_resolved' THEN #{average_value_key} END) as avg_resolution_time",
+                "AVG(CASE WHEN name = 'first_response' THEN #{average_value_key} END) as avg_first_response_time",
+                "AVG(CASE WHEN name = 'reply_time' THEN #{average_value_key} END) as avg_reply_time"
+              )
+              .group(group_by_key)
+              .index_by { |record| record.public_send(index_key) }
+
+    @resolved_count = results.transform_values(&:resolved_count)
+    @avg_resolution_time = results.transform_values(&:avg_resolution_time)
+    @avg_first_response_time = results.transform_values(&:avg_first_response_time)
+    @avg_reply_time = results.transform_values(&:avg_reply_time)
   end
 
   def reporting_events
@@ -24,14 +42,6 @@ class V2::Reports::BaseSummaryBuilder
     # Override this method
   end
 
-  def fetch_average_time(event_name)
-    get_grouped_average(reporting_events.where(name: event_name))
-  end
-
-  def fetch_resolved_count
-    reporting_events.where(name: 'conversation_resolved').group(group_by_key).count
-  end
-
   def group_by_key
     # Override this method
   end
@@ -40,10 +50,6 @@ class V2::Reports::BaseSummaryBuilder
     # Override this method
   end
 
-  def get_grouped_average(events)
-    events.group(group_by_key).average(average_value_key)
-  end
-
   def average_value_key
     ActiveModel::Type::Boolean.new.cast(params[:business_hours]).present? ? :value_in_business_hours : :value
   end

app/builders/v2/reports/inbox_summary_builder.rb

@@ -13,10 +13,7 @@ class V2::Reports::InboxSummaryBuilder < V2::Reports::BaseSummaryBuilder
 
   def load_data
     @conversations_count = fetch_conversations_count
-    @resolved_count = fetch_resolved_count
-    @avg_resolution_time = fetch_average_time('conversation_resolved')
-    @avg_first_response_time = fetch_average_time('first_response')
-    @avg_reply_time = fetch_average_time('reply_time')
+    load_reporting_events_data
   end
 
   def fetch_conversations_count