Feature: Daily personalized developer network digest

[sajeetharan]

Problem

Users can discover developers, but DevGlobe does not summarize what changed in their network since the previous visit.

Proposal

Send an opt-in daily digest containing a small number of meaningful changes from followed developers: rank movement, milestones, new top repositories, and profile updates. Reuse the explicit watchlist and personalized-feed data; never infer interests.

Acceptance criteria

• Signed-in users can opt into or out of the daily digest.
• Delivery contains only public events matching explicit follows.
• Emails are idempotent and contain a direct link to the personalized feed.
• Empty digests are not sent.
• Delivery, open, and return-click events are measurable without exposing private profile data.

Dependencies

• Feature: Developer and project watchlists with update notifications #113
• Feature: Personalized developer activity and watchlist feed #127
• Weekly ranking digest email #22