fix: update stale 'Postmark' comment to 'SMTP' in server.ts

The subscription error fallback comment still referenced Postmark
after the mailer migration. Update it to reflect the SMTP mailer.

src/server.ts

@@ -113,7 +113,7 @@ addRoute('post', '/subscription/email', async (req: Request, res: Response) => {
     const result = await registerSubscription({ pubkey, email, frequency })
     res.json(result)
   } catch (error: any) {
-    // If the error is just Postmark failing, the subscription was still created
+    // If the error is just SMTP failing, the subscription was still created
     // Look up the actual subscription key from the DB
     const sub = await getSubscriptionByPubkey(pubkey)
     if (sub) {