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/models/conversation.rb

@@ -167,6 +167,10 @@ class Conversation < ApplicationRecord
     agent_last_seen_at.present? ? messages.created_since(agent_last_seen_at) : messages
   end
 
+  def assignee_unread_messages
+    assignee_last_seen_at.present? ? messages.created_since(assignee_last_seen_at) : messages
+  end
+
   def unread_incoming_messages
     unread_messages.where(account_id: account_id).incoming.last(10)
   end