Signing keys

Mastodon 4.7.0 keeps local accounts' signing keys in keypairs, with the private half encrypted the way Rails encrypts columns. Give eunha the same secrets that Mastodon requires and it reads and writes that form, moving any keys still in the old accounts columns on startup:

ACTIVE_RECORD_ENCRYPTION__PRIMARY_KEY=...
ACTIVE_RECORD_ENCRYPTION__KEY_DERIVATION_SALT=...

Without them, keys stay in accounts.private_key, which upstream still reads (Keypair.from_legacy_account) — but a database whose keys have already moved cannot be signed with, and eunha says so loudly at startup.