feat(notifications): add scheduled digest email service

[ussyalfaks]

Summary

closes #1006

• Adds a digest_frequency column (daily | weekly | off, default off) to user_preferences so users can opt into digest emails
• Adds a nullable email column to users (required to resolve a send address per user)
• Introduces NotificationDigestState entity that tracks the last sent period per user (YYYY-MM-DD for daily, YYYY-WNN for weekly) to enforce idempotency
• Adds DigestService with two configurable cron jobs (registered dynamically via SchedulerRegistry) that fan out per-user digest emails each daily/weekly period
• Extends email-templates.ts with a digest template type that renders a bulleted HTML + plain-text summary of unread notifications

Behaviour

• Users with digest_frequency = 'off' or email_notifications = false are never emailed
• Users with no email on file are silently skipped
• Users with no unread activity in the window receive no email (no empty digests)
• Running the cron twice for the same period is a no-op — the period key is persisted on first send

Environment variables

Variable	Default	Description
DIGEST_ENABLED	true	Set to false to disable all digest jobs at startup
DIGEST_DAILY_HOUR_UTC	8	UTC hour the daily digest fires (0–23)
DIGEST_WEEKLY_DAY	1	Day of week for weekly digest (0=Sun … 6=Sat)

Test plan

• Set digest_frequency = 'daily' and email_notifications = true on a user with unread notifications — confirm one email queued per day
• Trigger the daily cron twice in the same UTC day — confirm second run sends nothing
• Set email_notifications = false — confirm no email sent
• Set digest_frequency = 'off' — confirm no email sent
• Leave email null on a user — confirm skipped silently
• User with zero unread notifications in the window — confirm no email queued
• Run migration against a clean DB and verify all three schema changes apply cleanly

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Project	Deployment	Actions	Updated (UTC)
insight-arena-4rll	Ready	Preview, Comment	Jun 23, 2026 5:09am

[Olowodarey]

@ussyalfaks great job clean code