Send welcome emails when a new user signs up (#691)

* Don't trigger any re-indexing on server initailization

* Integrate Resend to send welcome emails when a new user signs up

- Only send if this is the first time they've signed in
- Configure welcome email with basic styling, as more complex designs don't work and style tag did not work

pyproject.toml

@@ -94,6 +94,7 @@ prod = [
     "stripe == 7.3.0",
     "twilio == 8.11",
     "boto3 >= 1.34.57",
+    "resend >= 0.8.0",
 ]
 dev = [
     "khoj-assistant[prod]",