Add notification for new storyline created

[realproject7]

Summary

Currently only notifyNewPlot() exists as a notification trigger. Add a notification when a new storyline is created so users are alerted to new stories on PlotLink.

Implementation

• Add notifyNewStoryline() to lib/notifications.server.ts
• Title: e.g. New story published
• Body: e.g. "{title}" by {author} is now on PlotLink
• Target URL: plotlink.xyz/story/{storylineId}
• Notification ID: pl-new-storyline-{storylineId}
• Call from the backfill cron when a new storyline is first indexed

Acceptance Criteria

• notifyNewStoryline() function added to lib/notifications.server.ts
• Triggered from backfill cron on new storyline detection
• Notification received in Farcaster client with correct title, body, and deep link