Fix discovery to actually use negative affinity signals

Previously the engine was blind to dislikes/dismissals:
- _build_user_tag_profile only used liked/watched (positive only)
- dismiss_penalty was capped at 80% so hated content still surfaced
- _search_and_store had zero affinity filtering, any YouTube result entered the queue
- user_tag_affinity negative scores (written by dismiss/dislike) were never read

Now:
- _build_user_tag_profile reads directly from user_tag_affinity (positive + negative)
- _tag_relevance_score returns negative values, so disliked-tag channels score below zero and get dropped
- _search_and_store skips channels whose indexed videos match 3+ negatively-rated tags
- list_discovery post-filters channels already in the queue using the same neg-affinity check
- Removed the old _dismissed_channel_tags + dismiss_penalty (superseded)

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

backend/routers/discovery.py

@@ -56,9 +56,32 @@ def list_discovery(
             ORDER BY dq.score DESC
             LIMIT :limit OFFSET :offset
         """),
-        {"user_id": current_user.id, "limit": limit, "offset": offset},
+        {"user_id": current_user.id, "limit": limit * 3, "offset": offset},
     ).mappings().all()
 
+    # Load negative affinity tags and use them to filter channels already in the queue
+    neg_affinity = {
+        r["tag"] for r in db.execute(
+            text("SELECT tag FROM user_tag_affinity WHERE user_id = :user_id AND score < -2"),
+            {"user_id": current_user.id},
+        ).mappings().all()
+    }
+    if neg_affinity and rows:
+        channel_ids_csv = ",".join(str(r["channel_id"]) for r in rows)
+        vtag_rows = db.execute(
+            text(f"SELECT channel_id, tags FROM videos WHERE channel_id IN ({channel_ids_csv}) AND tags IS NOT NULL LIMIT 1000")
+        ).mappings().all()
+        neg_hit: dict[int, int] = {}
+        for vr in vtag_rows:
+            try:
+                for tag in json.loads(vr["tags"] or "[]"):
+                    if isinstance(tag, str) and tag.lower().strip() in neg_affinity:
+                        neg_hit[vr["channel_id"]] = neg_hit.get(vr["channel_id"], 0) + 1
+            except (json.JSONDecodeError, TypeError):
+                pass
+        rows = [r for r in rows if neg_hit.get(r["channel_id"], 0) < 3]
+
+    rows = rows[:limit]
     items = []
     for row in rows:
         row = dict(row)