Add endpoint for creating scheduled newsletters

[BhaveshKukreja29]

Description

Closes #126

This PR introduces the POST /create-newsletter endpoint, which allows authenticated users to schedule recurring newsletters. The implementation includes plan-based limits and sends a sample newsletter to whitelisted users upon creation.

The /user endpoint has also been updated to return newsletter usage stats.

Checklist

• I have created an issue for this change (not mandatory for small changes)
• My changes are to-the-point
• Code is styled as the rest of the codebase and linting passes
• I have tested my changes locally and they work as expected
• I have updated the documentation (if needed)
• I have reviewed the code myself once and I don't see any issues

Additional Notes

Changes by File

• mxgo/api.py:

  • Added the new /create-newsletter endpoint.
  • It checks user plan limits for the max number of newsletters and minimum schedule interval.
  • Updated the /user endpoint to include newsletter_usage in the response.

• mxgo/config.py:

  • Added NEWSLETTER_LIMITS_BY_PLAN to define the limits.

• mxgo/schemas.py:

  • Added CreateNewsletterRequest and CreateNewsletterResponse for the new endpoint.
  • Introduced ScheduleOptions to accept a user-friendly schedule (e.g., weekly on specific days) instead of a raw cron string.

• mxgo/utils.py:

  • Added convert_schedule_to_cron_list to convert the new ScheduleOptions into one or more cron expressions.
  • Moved and updated calculate_cron_interval to validate schedule frequency against plan limits.

• tests/test_api.py:

  • Added a new test suite for /create-newsletter.
  • Tests cover the success path, plan limit enforcement, and whitelist logic.

Request Response Structure

Request: POST /create-newsletter

{
  "prompt": "Weekly summary of AI advancements",
  "schedule": {
    "type": "RECURRING_WEEKLY",
    "recurring_weekly": {
      "days": ["friday"],
      "time": "09:00"
    }
  }
}

Success Response (200 OK):

{
  "is_scheduled": true,
  "is_whitelisted": true,
  "sample_email_sent": true,
  "scheduled_task_ids": [
    "c4a6b2e8-6e7b-4a1d-8c3e-0f1d5e2a3b4c"
  ]
}

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View logs

[satwikkansal]

There's a merge conflict with main branch, let's resolve that as well

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