Add email sink (SMTP or SendGrid)

[YuyangXueEd]

Summary

Add an email delivery sink so users can receive their daily digest directly in their inbox.

Why

Email is the most universally available notification channel. Many users don't have a Slack workspace or ServerChan account, but everyone has email. This is the highest-impact missing sink.

What to implement

A new sink at sinks/email/ that:

• Sends the daily digest as an email with both plain-text and HTML parts
• Supports two backends (contributor chooses one or both):
  • SMTP — works with Gmail, Outlook, Fastmail, self-hosted, etc.
  • SendGrid — simpler API, generous free tier
• Reads credentials from environment variables:
  • SMTP: SMTP_HOST, SMTP_PORT, SMTP_USER, SMTP_PASSWORD
  • SendGrid: SENDGRID_API_KEY
• Config options: to_address, from_address, subject_prefix, max_papers, max_hn, max_github

Config shape

sinks:
  email:
    enabled: false
    backend: smtp          # or "sendgrid"
    to: "you@example.com"
    from: "digest@example.com"
    subject_prefix: "[MyDailyUpdater]"
    max_papers: 5
    max_hn: 3
    max_github: 3

How to get started

1. Copy sinks/_template/ to sinks/email/
2. Look at sinks/slack/ as a reference for the delivery pattern
3. Register in sinks/__init__.py
4. Wire secrets into .github/workflows/daily.yml
5. Write tests — mock the SMTP connection or SendGrid client
6. Add sinks/email/README.md with setup instructions for both backends

Acceptance criteria

• sinks/email/__init__.py with EmailSink class
• sinks/email/README.md with setup instructions
• At least one backend (SMTP or SendGrid) fully working
• Registered in sinks/__init__.py
• Config block in config/sources.yaml (commented out)
• Secrets documented and wired into daily.yml
• At least one test
• pytest tests/ -q passes

[snakefood3232]

I'll add the email sink with both SMTP and SendGrid backends as outlined. I'd implement this by following your template approach, creating a dual-backend system that reads from environment variables. I can have a working PR with tests and documentation ready in about 8 hours—I've integrated both SMTP and SendGrid APIs in similar notification systems before.