appview/db: allow regenerating invite codes

users sometimes want to regenerate invite codes, but signups_inflight
never replaces the old value, since the email column has a unique
constraint. by using `or replace`, we can update the invite code.

this keeps the db in sync with whatever the user sees in their inbox.

Signed-off-by: oppiliappan <[e-mail]>

appview/db/signup.go

···
 )
 
 func AddInflightSignup(e Execer, signup models.InflightSignup) error {
-	query := `insert into signups_inflight (email, invite_code) values (?, ?)`
+	query := `insert or replace into signups_inflight (email, invite_code) values (?, ?)`
 	_, err := e.Exec(query, signup.Email, signup.InviteCode)
 	return err
 }