feat(API): user invitations and emails

api/pkg/apperr/messages.go

@@ -16,3 +16,6 @@ const VerificationRequiredErrorMessage = "You must verify your account to access
 const PasswordResetRequestedMessage = "If an account exists for that email, a password reset link has been sent."
 const PasswordResetFailedErrorMessage = "The password reset code is invalid or has expired."
 const TooManyRequestsErrorMessage = "Too many requests. Please try again later."
+const EmailHasAccountErrorMessage = "This address already has an account."
+const PendingInviteErrorMessage = "There's already a pending invite for this address."
+const InviteNotSentErrorMessage = "Couldn't send the invite. Try again in a moment."