updated mailbox app password

internal/db/mailbox_models.go

@@ -79,7 +79,8 @@ type MailboxFilterRule struct {
 }
 
 // MailboxAppPassword is the only credential an IMAP/SMTP client ever uses. Plaintext
-// is shown once at creation and never stored.
+// is shown once at creation and never stored. ExpiresAt is nil for a password that
+// never expires (the default).
 type MailboxAppPassword struct {
 	ID           int64
 	MailboxID    int64
@@ -88,6 +89,7 @@ type MailboxAppPassword struct {
 	IsActive     bool
 	CreatedAt    time.Time
 	LastUsedAt   *time.Time
+	ExpiresAt    *time.Time
 }
 
 // MailboxMessage is one stored message. CachedFrom/CachedSubject are plaintext by