perf: reuse resolved count for reopen rate (#15122)

This improves the Captain overview by loading reporting metrics and FAQ
stats from separate endpoints. Range changes now refresh only the
metrics, while reopen-rate calculation reuses the resolved conversation
count to avoid redundant database queries.

## What changed

- Split Captain overview metrics and FAQ stats into separate APIs.
- Fetch FAQ stats independently from range-based metrics.
- Reuse resolved conversation totals when calculating reopen rate.
- Skip the reopen query when there are no resolved conversations.

spec/enterprise/builders/captain/assistant_stats_builder_spec.rb

@@ -27,7 +27,7 @@ RSpec.describe Captain::AssistantStatsBuilder do
 
       expect(metrics.keys).to contain_exactly(
         :conversations_handled, :auto_resolution_rate, :handoff_rate,
-        :hours_saved, :reopen_rate, :conversation_depth, :knowledge
+        :hours_saved, :reopen_rate, :conversation_depth
       )
       expect(metrics[:conversations_handled]).to include(:current, :previous, :trend)
     end
@@ -229,7 +229,7 @@ RSpec.describe Captain::AssistantStatsBuilder do
     end
   end
 
-  describe '#metrics knowledge' do
+  describe '#faq_stats' do
     before do
       create_list(:captain_assistant_response, 3, assistant: assistant, account: account, status: :approved)
       create(:captain_assistant_response, assistant: assistant, account: account, status: :pending)
@@ -237,7 +237,7 @@ RSpec.describe Captain::AssistantStatsBuilder do
     end
 
     it 'returns approved, pending, document counts and coverage' do
-      knowledge = described_class.new(assistant, '30').metrics[:knowledge]
+      knowledge = described_class.new(assistant).faq_stats
 
       expect(knowledge).to eq(approved: 3, pending: 1, documents: 2, coverage: 75)
     end
@@ -245,7 +245,7 @@ RSpec.describe Captain::AssistantStatsBuilder do
     it 'reports zero coverage when there are no responses' do
       Captain::AssistantResponse.where(assistant: assistant).delete_all
 
-      knowledge = described_class.new(assistant, '30').metrics[:knowledge]
+      knowledge = described_class.new(assistant).faq_stats
 
       expect(knowledge[:coverage]).to eq(0)
     end

spec/enterprise/policies/captain/assistant_policy_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe Captain::AssistantPolicy, type: :policy do
   let(:administrator_context) { { user: administrator, account: account, account_user: account.account_users.first } }
   let(:agent_context) { { user: agent, account: account, account_user: account.account_users.first } }
 
-  permissions :index?, :show?, :playground? do
+  permissions :index?, :show?, :playground?, :metrics?, :faq_stats? do
     context 'when administrator' do
       it { expect(assistant_policy).to permit(administrator_context, assistant) }
     end