Use environment variable to set sender email of auth link emails (#907)

Set sender email using `RESEND_EMAIL` environment variable for magic link sent via Resend API for authentication . It was previously hard-coded. This prevented hosting Khoj on other domains. 

Resolves #908

src/khoj/routers/email.py

@@ -44,7 +44,12 @@ async def send_magic_link_email(email, unique_id, host):
     html_content = template.render(link=f"{host}auth/magic?code={unique_id}")
 
     resend.Emails.send(
-        {"sender": "noreply@khoj.dev", "to": email, "subject": "Your Sign-In Link for Khoj 🚀", "html": html_content}
+        {
+            "sender": os.environ.get("RESEND_EMAIL", "noreply@khoj.dev"),
+            "to": email,
+            "subject": "Your Sign-In Link for Khoj 🚀",
+            "html": html_content,
+        }
     )