notifications: announcement channel selector + proper re-engagement emails

[dylanjeffers]

Summary

Two related changes that make /internal/send-notification announcements (including the re-engagement / inactivity campaigns the notifications-dashboard cron sends) deliver good emails, not just push:

1. Channel selector (c274d84): /internal/send-notification accepts an optional notificationTypes of email | push | both, stored as notification_channels on the announcement row and honored by the mapper. Defaults to both for back-compat.
2. Email rendering fixes (91cbb9f): announcement emails previously reused the generic unread-notification shell and discarded the campaign's heading, CTA link, and artwork — the subject was always "N unread notifications on Audius".

Context

Re-engagement notifications are the same announcement notification type as one-off announcements (the cron inserts type: 'announcement' via the endpoint; the mapper dispatches on type == 'announcement'). The dashboard's send-reengagement cron passes the same fields a regular announcement does — title (heading), body, route (CTA), image_url — but the email side dropped all but the body.

Email fixes (all 5)

• Heading — getTitle() now renders data.title for announcements (was hardcoded null).
• CTA route — WrapLink builds an absolute audius.co deep-link from the notification's route instead of the hardcoded /feed?openNotifications=true.
• Artwork — the announcement body now renders image_url.
• Subject — single-announcement emails use the heading instead of the generic unread-count subject.
• Back-compat — every field is optional; rows without them fall back to the prior behavior (no heading/image, feed link, generic subject).

Testing

• tsc --noEmit clean for apps/notifications/src (pre-existing @types/babel__traverse node_modules errors are unrelated).

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