feat: Add "Reply To" address configuration for emails

[sahare77]

Description
When sending emails via the Gmail API, all replies go directly to the sender. Users might want to specify a custom Reply-To header for campaigns.

Steps to Reproduce

1. Go to Campaign Builder.
2. Add an Email step.
3. Notice there's no way to set a "Reply-To" address.

Expected Behavior
Add a reply_to field in the Campaign or Email Step model. When drafting the raw email for the Gmail API, include the Reply-To header.

Implementation Hints

# backend/campaigns/services.py
from email.message import EmailMessage

msg = EmailMessage()
msg['To'] = lead.email
msg['From'] = connected_account.email
if step.reply_to:
    msg['Reply-To'] = step.reply_to

[sahare77]

Hello @Kuldeeep18, I want to work on this issue. Please assign it to me under GSSoC 2026.

[Swatigithub2004]

Hi! I have reviewed the issue and understand the required backend changes. I would like to work on this issue under GSSoC 2026. I can implement the Reply-To field, update the email generation logic, and test the functionality. Could you please assign this issue to me? Thank you!

[aaniya22]

pls assign this to me under gssoc

[sahare77]

/assign under gssoc'26