Remind operators to run the backfill again after the deploy

The seed migrations run at the start of a rolling deploy, while old API
servers are still serving traffic. Their old cleanup code can still
delete start events that the new code depends on. Only running 'rake
db:was_running_backfill' once after the deploy finishes closes that
window. Until now this instruction lived only in the docs and the rake
task description. Now the last seed migration also logs it, so it shows
up in the migration output that operators see during the deploy.

Author: joyvuu-dave

db/migrations/20260601120300_seed_was_running_task_usage_events.rb

@@ -13,6 +13,14 @@
       VCAP::WasRunningBackfill.with_advisory_lock(self, wait: true) do
         VCAP::WasRunningBackfill.seed_task_usage_events(self, logger)
       end
+      # This is the last of the three seed migrations, so remind the operator
+      # here. The migrations run at the start of a rolling deploy, while old
+      # API servers are still serving traffic; their old cleanup code can
+      # still delete start events that the new code depends on. The seed
+      # cannot see the future, so the operator has to close that window by
+      # running the backfill once more after the deploy finishes.
+      logger.info("WAS_RUNNING usage event backfill complete. If old API servers were still serving traffic during this deploy, run 'rake db:was_running_backfill' " \
+                  'once after the deploy finishes to repair anything they changed in the meantime.')
     end
   end