Decide the shape of per-protocol capability state on Account

[Hohnik]

Part of #18

Question

What exactly does an Account store about each protocol's usability, and where does it live?

Options span: a pair of booleans (imap_verified / smtp_verified); a status enum per protocol (unknown / ok / failed) plus last_error and last_checked_at; or a separate capabilities table keyed by (account, protocol).

Sub-questions this must settle, because every other ticket on the map reads the answer:

• Is the server's raw error text persisted for display, or only a status?
• Is there a meaningful unknown state (never checked) distinct from failed?
• Is the timestamp of the last check needed, or does nothing depend on check age?
• Does sent_folder's "nullable, unset until the user acts" pattern apply here too?