Overhaul For You feed ranking and freshness

Ranking improvements:
- Wider candidate pool (4x limit) with ±12pt score perturbation so
  same-score videos shuffle differently each load
- Recent channel engagement signal: channels watched in past 30 days
  get a +4pts/watch boost
- Bail penalty: -25pts for videos started but abandoned before 20%
- Impression penalty: -3pts per prior feed appearance (capped at 10),
  so repeatedly-skipped videos sink naturally
- rn cap raised to 5 for more candidates; Python-side sampling picks top limit

Feed UX:
- Reshuffle button now available on For You (ranked) mode, not just Explore
- shuffleKey now always included in query key (not just random mode)
- Ranked mode staleTime reduced from 10min to 90s

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/main.py

@@ -130,6 +130,7 @@ def on_startup():
             note TEXT DEFAULT '',
             created_at DATETIME DEFAULT CURRENT_TIMESTAMP
         )""",
+        "ALTER TABLE user_videos ADD COLUMN feed_shown_count INTEGER NOT NULL DEFAULT 0",
     ]:
         try:
             db.execute(text(col_sql))