feat: Add script to clean orphaned reference records

Instances that accumulated project managers, favorites, board memberships
or board subscriptions pointing at a project or board that no longer exists
had no way to find or clear them.

The script reports by default and only deletes with `--apply`, which first
asks whether a backup exists and refuses outright without a terminal to ask
on. It removes nothing but reference rows: structural orphans such as a
board without a project carry user content, and a membership whose
denormalized project is stale still grants real access to an existing
board, so both are reported and left alone.

server/package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "build": "node build.js",
     "console": "dotenv sails console",
+    "db:clean-orphaned-records": "node db/clean-orphaned-records.js",
     "db:create-admin-user": "node db/create-admin-user.js",
     "db:init": "node db/init.js",
     "db:migrate": "knex migrate:latest --cwd db",