Action due-date reminder emails with per-user configurable timing

[jhodapp]

Summary

As a user (coach or coachee), I want to receive a reminder email before an action's due date approaches. Each user should be able to configure when (and whether) these reminders are sent from their Settings page.

User story

• As a coach or coachee
• I want a notification reminder email when an action I own has an upcoming due date
• So that I can stay on top of my commitments without manually tracking them

Requirements

Notification

• Send an email reminder to the action's owner ahead of the action's due_by date
• Triggered per-action (one user's setting drives reminders for actions they own)

User Settings (per-user)

• Add a section under the Settings page (alongside existing user preferences)
• Configurable fields per user:
  • Enable/disable action due-date reminder emails
  • Lead time before due date (e.g. 1 day, 3 days, 1 week — exact options TBD)
• Setting is stored per-user and applies to all of that user's actions

Out of scope

• In-app / push notifications (email only for now)
• Reminders for other entity types (sessions, agreements, etc.)

Open questions

• Exact set of lead-time options
• Default value for new users (opt-in vs opt-out)
• Backend delivery mechanism (cron, scheduled job) — coordinate with backend repo