feat: add For You feed (extended/follows + trending + hashtags)

[barrydeen]

Summary

• New FOR_YOU feed type, set as the default for new installs (returning users keep their saved feed type)
• Blends three sources into one sorted stream: extended/follow feed, top-reactions + top-replies trending notes, and notes for all followed hashtags
• Trending notes (fast relays) fire inline; hashtag notes (slow search relay) batch 10 at a time over a 24h window and stream in after the fact

Implementation notes

• All supplementary events funnel through EventRepository.addEvent(), reusing the existing dedup (seenEventIds) and binary-insert-by-created_at logic — no separate stream or merge layer
• Supplementary fetches self-close on EOSE + short drain, so switching tabs doesn't leak subscriptions
• Silent fallback when the social graph isn't computed yet — For You quietly uses follows-only instead of blocking on a loader
• loadMore only paginates the main author subscription; trending/hashtag batches are initial-load only
• restoreSavedFeedType updated so previously-saved FOLLOWS users still land on FOLLOWS on next boot

Test plan

• New install: lands on For You by default
• Returning user with saved FOLLOWS/EXTENDED/etc: still restores correctly
• Fresh account without social graph: For You silently shows follow feed (no loader)
• Trending notes appear alongside follow content within ~1-3s
• With 15+ followed hashtags: tagged notes stream in, properly sorted and deduped
• With no followed hashtags: only trending supplements the follow feed, no search-relay REQs
• Toggle FOR_YOU → FOLLOWS → FOR_YOU: supplementary fetches re-fire, no orphaned subscriptions
• Scroll to bottom: loadMore fetches older follow/extended only
• Trending relay unreachable: feed still loads follow/extended content