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
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