feat(auth): invite links for several people, invite by email, registration with approval or email domains

- Invites: link /?invite=CODE, copy link, send by email, max uses with counter
- Registration modes closed | approval | domain | domain+approval | open, waiting list with approve/reject
- REGISTRATION_MODE / REGISTRATION_DOMAINS lock the admin setting; ALLOW_REGISTRATION=true is honoured

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/register_limits_test.go

@@ -12,7 +12,7 @@ import (
 func TestInviteCodeRedeemedOnlyOnce(t *testing.T) {
 	srv, store, _ := setupTestServer(t)
 	router := srv.Router()
-	inv, err := store.CreateUserInvite("", "", "member", "", 7)
+	inv, err := store.CreateUserInvite("", "", "member", "", 7, 1)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -46,7 +46,7 @@ func TestInviteCodeRedeemedOnlyOnce(t *testing.T) {
 func TestInviteReleasedWhenAccountFails(t *testing.T) {
 	srv, store, _ := setupTestServer(t)
 	router := srv.Router()
-	inv, _ := store.CreateUserInvite("", "", "member", "", 7)
+	inv, _ := store.CreateUserInvite("", "", "member", "", 7, 1)
 	_, _ = store.CreateUser("taken@example.com", "hash", "Taken", "member")
 	body := func(email string) string {
 		return `{"email":"` + email + `","password":"password123","inviteCode":"` + inv.Code + `"}`