feat: UTM digest tracking + AE last-viewed roster

[m1lestones]

What this does

Closes the loop on whether AEs are actually opening the Monday digest.

• DigestTracker — fixed to read ae_id from URL params instead of hardcoding Danielle's ID. Fires on ?utm_source=digest&ae_id=<uuid>&digest_id=<uuid>.
• /api/digest-view — new Next.js route. Stores views in Supabase digest_views table (upsert by ae_id) and also forwards to your /api/v1/digest-views endpoint silently.
• DigestRoster — Danielle sees a "Digest · Last Opened" panel on the home page showing each AE with a green dot + time ago, or "not opened".

What Joel needs to do

1. Run this in Supabase:

CREATE TABLE IF NOT EXISTS digest_views (
  ae_id uuid PRIMARY KEY,
  digest_id text,
  viewed_at timestamptz DEFAULT now()
);

2. In format_weekly_digest(), append UTM params to every dashboard link:

?utm_source=digest&digest_id={digest_id}&ae_id={ae_user_id}

That's it — frontend handles everything else. The Supabase write works independently of your endpoint, so this is safe to merge before your digest endpoint is live.

No dependencies on feat/mobile-polish

This PR is based off main and can be merged in any order.

🤖 Generated with Claude Code

[m1lestones]

Closing — merged into feat/mobile-polish instead to keep everything together before Demo Day.