Invite players and verify accounts

Players join with the ladder's invite link (/l/<ladder>/join?c=CODE). It has three steps:

1. They type the name you know them by and their Lichess and/or chess.com username. I check the accounts exist.
2. They get a code like CL7KX4MP and paste it into their profile: on Lichess in Biography or Location, on chess.com in Location.
3. They press "Check now". When I find the code, they're on the ladder and can delete it from their profile.

A profile change on chess.com is not visible to other sites at once. If both accounts are entered and only one is found, the player can join with that one and add the other later with the same link.

Why the code

It proves the account belongs to the person joining, so nobody can put someone else's account on a ladder. Verified players get the verified mark, can appear on public ladders, and feed the rating comparison when they verify both sites.

Young players

Use a first name and last initial, keep the ladder unlisted, and send the link to parents rather than children. Club Ladder hosts no games or chat.

New link

If an invite link gets passed around, make a new one on the admin page. The old one stops working; players already on the ladder stay.