Add migration script to fix misplaced comment events (#2487)

Finds comment events that are on the wrong board (after a card move)
and updates them to the correct board.

script/migrations/20260204-fix-misplaced-comment-events.rb

@@ -0,0 +1,59 @@
+# Fix comment events that are on the wrong board after a card move.
+#
+# See https://github.com/basecamp/fizzy/pull/2486
+#
+# Usage:
+#   bin/rails runner script/migrations/20260204-fix-misplaced-comment-events.rb           # dry run
+#   bin/rails runner script/migrations/20260204-fix-misplaced-comment-events.rb --fix     # actually fix
+#
+dry_run = !ARGV.include?("--fix")
+
+puts dry_run ? "DRY RUN - no changes will be made\n\n" : "FIXING misplaced events\n\n"
+
+misplaced_events = Event
+  .where(eventable_type: "Comment")
+  .joins("INNER JOIN comments ON comments.id = events.eventable_id")
+  .joins("INNER JOIN cards ON cards.id = comments.card_id")
+  .where("events.board_id != cards.board_id")
+
+total = misplaced_events.count
+puts "Found #{total} misplaced comment events\n\n"
+
+if total.zero?
+  puts "Nothing to fix!"
+  exit
+end
+
+fixed = 0
+skipped = 0
+
+misplaced_events.find_each.with_index do |event, index|
+  comment = event.eventable
+  card = comment&.card
+  old_board = event.board
+  new_board = card&.board
+
+  puts "[#{index + 1}/#{total}] Event #{event.id}"
+
+  if card.nil? || new_board.nil?
+    puts "  Skipping - orphaned data (comment or card deleted)"
+    skipped += 1
+    puts
+    next
+  end
+
+  puts "  Card ##{card.number}: #{card.title.truncate(40)}"
+  puts "  Moving from board '#{old_board&.name || 'nil'}' to '#{new_board.name}'"
+
+  if dry_run
+    puts "  (skipped - dry run)"
+  else
+    event.update!(board: new_board)
+    fixed += 1
+    puts "  Fixed!"
+  end
+
+  puts
+end
+
+puts "Done. #{dry_run ? "Run with --fix to apply changes." : "Fixed #{fixed} events."} (#{skipped} skipped)"