Consider handling `db.sqlite3` failure cases better in `build-dbs-for-local-development`

I was trying this out today and noticed:

• it absolutely requires that a db.sqlite3 exists before running
  • you have to touch db.sqlite3 if it doesn't exist
  • if the command subsequently fails, you have to touch db.sqlite3 again, because the empty file you create then ends up as db.sqlite3.backup
• if you do have an existing db.sqlite3.backup, it overwrites this without prompting
  • failing loudly if the backup exists already is one option
  • another option might be creating a file with a unique suffix to avoid overwriting the existing file

Start at the build-dbs-for-local-development entry point and reproduce the missing db.sqlite3 and existing db.sqlite3.backup cases described in the issue. Done means the command handles failures without requiring repeated manual file creation and does not silently overwrite an existing backup.