fix: remove dead predexon feed, hide empty feeds tab

[Flotapponnier]

Summary

• Remove Predexon from `PM_DATA_FEEDS` and `PM_FEED_META`: no harness has measured it since pm-data-freshness was retired (2026-07). Showing a row of "..." is misleading.
• Hide "Data feeds" tab in `pm-hub-tabs.tsx` when `feedsCount === 0` (was showing "Data feeds 0").
• Summary card on /prediction-markets now shows "4" instead of "4 + 0" when no feeds are tracked.
• Remove predexon from the PM hub pill list in `products/[slug]/page.tsx` (it had no product page — was 404ing).

Test plan

• /prediction-markets shows 4 venues, no "Data feeds" tab, summary card shows "4"
• /products/polymarket, /kalshi, /limitless, /myriad still have the "View on /prediction-markets" pill
• No 404 from clicking a dead feed link