perf(conversations): throttle agent_last_seen_at updates to reduce DB load (#13355)

High-traffic accounts generate excessive database writes due to agents
frequently switching between conversations. The update_last_seen
endpoint was being called every time an agent loaded a conversation,
resulting in unnecessary updates to agent_last_seen_at and
assignee_last_seen_at even when there were no new messages to mark as
read.

#### Solution
Implemented throttling for the update_last_seen endpoint:

**Unread messages present:**
- Updates immediately without throttling to maintain accurate
read/unread state
- Uses assignee_unread_messages for assignees, unread_messages for other
agents

**No unread messages:**
- Throttles updates to once per hour per conversation
- Checks if agent_last_seen_at is older than 1 hour before updating
- For assignees, checks both agent_last_seen_at AND
assignee_last_seen_at - updates if either timestamp is old
- Skips DB write if all relevant timestamps were updated within the last
hour

- Consolidated two separate update_column calls into a single
update_columns call to reduce DB queries

app/controllers/api/v1/accounts/conversations_controller.rb

@@ -110,6 +110,15 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
   end
 
   def update_last_seen
+    # High-traffic accounts generate excessive DB writes when agents frequently switch between conversations.
+    # Throttle last_seen updates to once per hour when there are no unread messages to reduce DB load.
+    # Always update immediately if there are unread messages to maintain accurate read/unread state.
+    return update_last_seen_on_conversation(DateTime.now.utc, true) if assignee? && @conversation.assignee_unread_messages.any?
+    return update_last_seen_on_conversation(DateTime.now.utc, false) if !assignee? && @conversation.unread_messages.any?
+
+    # No unread messages - apply throttling to limit DB writes
+    return unless should_update_last_seen?
+
     update_last_seen_on_conversation(DateTime.now.utc, assignee?)
   end
 
@@ -142,12 +151,25 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
   end
 
   def update_last_seen_on_conversation(last_seen_at, update_assignee)
+    updates = { agent_last_seen_at: last_seen_at }
+    updates[:assignee_last_seen_at] = last_seen_at if update_assignee.present?
+
     # rubocop:disable Rails/SkipsModelValidations
-    @conversation.update_column(:agent_last_seen_at, last_seen_at)
-    @conversation.update_column(:assignee_last_seen_at, last_seen_at) if update_assignee.present?
+    @conversation.update_columns(updates)
     # rubocop:enable Rails/SkipsModelValidations
   end
 
+  def should_update_last_seen?
+    # Update if at least one relevant timestamp is older than 1 hour or not set
+    # This prevents redundant DB writes when agents repeatedly view the same conversation
+    agent_needs_update = @conversation.agent_last_seen_at.blank? || @conversation.agent_last_seen_at < 1.hour.ago
+    return agent_needs_update unless assignee?
+
+    # For assignees, check both timestamps - update if either is old
+    assignee_needs_update = @conversation.assignee_last_seen_at.blank? || @conversation.assignee_last_seen_at < 1.hour.ago
+    agent_needs_update || assignee_needs_update
+  end
+
   def set_conversation_status
     @conversation.status = params[:status]
     @conversation.snoozed_until = parse_date_time(params[:snoozed_until].to_s) if params[:snoozed_until]