Add shared registration invite helper and use it for admin-created users

src/lib/server/registration-invite.ts

@@ -0,0 +1,18 @@
+import { auth } from '$lib/auth';
+
+/**
+ * Email an invited user a link to set their password and finish their profile.
+ *
+ * Reuses Better Auth's password-reset token flow; the `invite=1` marker makes
+ * the reset-password hook send the `registration-email` template instead of the
+ * usual "you requested a reset" email. The link lands on /reset-password, and
+ * after setting a password the user logs in and is sent to /profile/setup.
+ */
+export async function sendRegistrationInvite(email: string): Promise<void> {
+	await auth.api.requestPasswordReset({
+		body: {
+			email,
+			redirectTo: '/reset-password?invite=1'
+		}
+	});
+}