perf: use update_columns for contact last_activity_at

Replace `sender.update(last_activity_at:)` with `update_columns` in
Message#update_contact_activity. This runs on every incoming message
(3.3M calls/day) and the full ActiveRecord update lifecycle is
unnecessary for a single timestamp write:

- Skips before_save :sync_contact_attributes (only relevant for
email/phone/identifier changes, not last_activity_at)
- Skips after_update_commit :dispatch_update_event which fires
CONTACT_UPDATED → ActionCable broadcasts, webhook deliveries,
and LeadSquared hooks on every incoming message with no
meaningful data change
- Eliminates 192K seconds/day of DB time from contact UPDATEs
that trigger full validation and callback chains

app/models/message.rb

@@ -318,7 +318,9 @@ class Message < ApplicationRecord
   end
 
   def update_contact_activity
-    sender.update(last_activity_at: DateTime.now) if sender.is_a?(Contact)
+    # rubocop:disable Rails/SkipsModelValidations
+    sender.update_columns(last_activity_at: DateTime.now) if sender.is_a?(Contact)
+    # rubocop:enable Rails/SkipsModelValidations
   end
 
   def update_waiting_since