feat: Add an API to support querying metrics by ChannelType (#13255)

This API gives you how many conversations exist per channel, broken down
by status in a given time period. The max time period is capped to 6
months for now.

**Input Params:**
- **since:** Unix timestamp (seconds) - start of date range
- **until:** Unix timestamp (seconds) - end of date range


**Response Payload:**

```json
{
  "Channel::Sms": {
    "resolved": 85,
    "snoozed": 10,
    "open": 5,
    "pending": 5,
    "total": 100
  },
  "Channel::Email": {
    "resolved": 72,
    "snoozed": 15,
    "open": 13,
    "pending": 13,
    "total": 100
  },
  "Channel::WebWidget": {
    "resolved": 90,
    "snoozed": 7,
    "open": 3,
    "pending": 3,
    "total": 100
  }
}
```

**Definitons:**
resolved = Number of conversations created within the selected time
period that are currently marked as resolved.
snoozed = Number of conversations created within the selected time
period that are currently marked as snoozed.
pending = Number of conversations created within the selected time
period that are currently marked as pending.
open = Number of conversations created within the selected time period
that are currently open.
total = Total number of conversations created within the selected time
period, across all statuses.

app/builders/v2/reports/channel_summary_builder.rb

@@ -0,0 +1,38 @@
+class V2::Reports::ChannelSummaryBuilder
+  include DateRangeHelper
+
+  pattr_initialize [:account!, :params!]
+
+  def build
+    conversations_by_channel_and_status.transform_values { |status_counts| build_channel_stats(status_counts) }
+  end
+
+  private
+
+  def conversations_by_channel_and_status
+    account.conversations
+           .joins(:inbox)
+           .where(created_at: range)
+           .group('inboxes.channel_type', 'conversations.status')
+           .count
+           .each_with_object({}) do |((channel_type, status), count), grouped|
+      grouped[channel_type] ||= {}
+      grouped[channel_type][status] = count
+    end
+  end
+
+  def build_channel_stats(status_counts)
+    open_count = status_counts['open'] || 0
+    resolved_count = status_counts['resolved'] || 0
+    pending_count = status_counts['pending'] || 0
+    snoozed_count = status_counts['snoozed'] || 0
+
+    {
+      open: open_count,
+      resolved: resolved_count,
+      pending: pending_count,
+      snoozed: snoozed_count,
+      total: open_count + resolved_count + pending_count + snoozed_count
+    }
+  end
+end