amal66 puts connector credentials behind a stronger lock

amal66 proposed encrypting the credentials held for third-party connectors, so a copied database backup or exposed read-only replica would not automatically hand over access to connected accounts.

The design used a separate protective key for each stored credential and fresh randomness each time it was saved. It also checks whether encrypted data has been altered, refusing to use it if it has been tampered with. Existing records could still be read, avoiding a disruptive all-at-once migration, and the connector experience would stay the same for users.

This pull request was closed without being merged, so these protections did not land through this work.