Allow companion choice to persist at account level, with reroll/delete options

Currently the companion is computed locally from hash(userId + salt) where the salt is hardcoded in the binary. This means:

• Every fresh install produces the same default buddy
• Any customization requires patching the binary on each machine
• Updates reset the companion back to the default
• There is no way to delete or reroll your buddy through /buddy

Request:

1. Store the user's companion (species, rarity, stats, name) server-side on their account, so it follows them across all machines automatically
2. Add a reroll option in /buddy to regenerate the companion
3. Add a delete option in /buddy to remove the companion entirely

Users who invest time in their companion shouldn't lose it on every new install or update. And users unhappy with their default roll should be able to try again or opt out.