feat(account): restore the confirm deeplink token (domain layer)

Re-apply the deeplink backend deferred out of PR1a: migration 00006 adds
email_confirmations.purpose + link_token_hash (hand-edited jet), each issued code
now carries an opaque 256-bit token (only its SHA-256 stored), and ConfirmByToken
resolves a token to a login (confirm + clear guest) or a link (attach when free,
signal merge when owned elsewhere). issueCode now embeds the /app/#/confirm/<token>
link in the email. The confirm endpoint, gateway RPC and SPA route follow.

backend/internal/postgres/jet/backend/model/email_confirmations.go

@@ -21,4 +21,6 @@ type EmailConfirmations struct {
 	Attempts       int16
 	ConsumedAt     *time.Time
 	CreatedAt      time.Time
+	Purpose        string
+	LinkTokenHash  *string
 }